The Region Menu is where the user can tweak its regions, the parts of audio or MIDI that sit on the timeline.

Insert Region from Region List: If a region is selected in the Editor List, add it at the Edit point
Play: Starts playback at the beginning of the selected region(s), and stops at its(their) end
Loop: Creates a loop range on the selected region's boundaries, and starts the looped playback
Rename…: Changes the name of the region, that appears in its top left area
Properties…: Shows the Region properties window, that displays detailed information about the region and allow for some modifications
Loudness Analysis…: Shows the Audio Report/Analysis window, that displays detailed dBFS information as well as a spectrogram (dBFS of frequency against time)
Spectral Analysis…: Shows the Audio Report/Analysis window, that displays a integrated spectral view of the region (dBFS agaisnt frequency)
Edit
Combine: Creates a new region by joining the selected audio regions in the same track, and replaces those region with the newly created compound. The same rules are applied to create the compound as for playback regarding e.g. layering
Uncombine: Splits back the compound created by combining into its original audio regions
Pitch Shift…: Changes the tune of the audio region, by octave, semitones or percentage, based on spectral analysis. Optionaly, and if they have been set for the region, preserves the formants
Split/Separate: Cuts the selected regions at the Edit point, separating them in two regions
Split at Percussion Onset: Allows splitting the selected regions on its PErcussion Onsets marker as set by the Rhythm Ferret (Not usable as of 5.5)
Make Mono Regions: Creates mono regions out of a stereo or multichannel region by splitting it into its discrete channels. The created regions are added to the Editor List
Close Gaps: Extends (or reduces) the selected regions to be perfecltly aligned. Optionnaly, sets up a crossfade duration, or a pull-back (spacing between regions)
Place Transient: Places a transient at the Edit Point. Used e.g. for the Pitch Shift… action
Rhythm Ferret…: Opens the Rhythm Ferret which is a powerfull tool to sequence audio files
Strip Silence…: Opens the Strip Silence window which is a very handy tool to remove all audio under a user-chosen threshold (with a preview)
Reverse: Mirrors the audio horizontally
Layering
Raise to Top: On overlapping regions, puts the selected one(s) on top
Raise: On overlapping region, makes the selected one(s) one layer higher
Lower: Makes the selected region(s) one layer lower
Lower to Bottom: Sends the selected region to the background
MIDI
Transpose…: On a MIDI region, shows the Transpose MIDI window, allowing to shift the pitch of the whole MIDI region by ± n semitones or octaves
Insert Patch Change…: Inserts a patch change at the Edit Point, allowing a change of patch, channel, program and/or bank
Quantize…: Shows the Quantize window, allowing to perfectly align the MIDI notes to the musical grid
Legatize: Shortens or elongates the MIDI notes to make them perfectly sequentials, i.e. the end of a note is the start of the following one
Remove Overlap: Shortens or elongates the MIDI notes to make them perfectly sequentials, i.e. the end of a note is the start of the following one
Transform…: Transform window, that allows for mathematical operations on the midi notes
Unlink from Other copies: Makes the selected MIDI region independant, e.g. editing this region won't affect any other one.
List Editor…: Shows the List Editor which sequentially lists all the MIDI events in the region, and allows for precise modifications
Gain
[] Opaque: When checked, makes the region opaque audio-wise, i.e., the underlying regions won't be audible
[] Mute: When checked, mutes only the selected region on the track, without muting the track. The muted regions will have !! prepended to their name and will be semi-transparent
Normalize…: Shows the Normalize region dialog, which allows to scale the region level by setting its maximum level, optionaly constraining the RMS
Boost Gain: Increases the gain on the selected region by boosting the audio, without touching the enveloppe or automation
Cut Gain: Reduces the gain without touching the enveloppe or automation
Reset Envelope: If the gain enveloppe has been edited, resets it to its initial value (constant at 0 dB)
[] Envelope Active: When unchecked, disables any enveloppe editing that has been made. The enveloppe will be displayed in yellow instead of green.
Position
Move to Original Position: Moves the region where it was initially recorded or inserted in the session
Snap Position to Grid: If the Grid Mode is set to Grid, snaps the region to the nearest grid line
[] Lock: Blocks the selected regions at their current positions in time and tracks, avoiding any movement on the timeline. The region name will be surrounded by > and < brackets
[] Glue to Bars and Beats: Locks the region position to relative to the musical grid, i.e. a change of tempo will move the region to keep it on the same bar/beat
[] Lock to Video: Same as above, relative to the position in the video
Set Sync Position: Creates or move the Sync position, i.e. the point of the region that will be aligned or snapped to the grid, and that is (by default) the beggining of the region.
Remove Sync: Removes any user defined Sync point, and resets the sync position to the beginning of the region
Nudge Later: Moves the region to the right by the amount shown in the nudge timer
Nudge Earlier: Same as above, to the left
Nudge Later by Capture Offset: Moves the region to the right by the capture latency computed by ardour based on the user's settings regarding latency
Nudge Earlier by Capture Offset: Same as above, to the left
Sequence Regions: Puts the selected regions one after the other, so that the end of one region is the beggining of the next one, removing any overlap or silence. The reference point is the earliest region.
Trim
Trim Start at Edit Point: If the Edit Point is within the region boundaries, shortens the region to align its start with the Edit Point
Trim End at Edit Point: Same as above, for the end of the region
Trim to Loop: Uses both the start and end Loop markers to shorten the region
Trim to Punch: Same as above with the Punch markers
Trim to Previous: On overlapping regions, shortens the selected one so that the previous region is complete, i.e. the new start point for the selected region is the end point of the previous region on the timeline
Trim to Next: Same as above, with the end of the selected region aligned to the start of the following one.
Ranges
Set Loop Range: Creates a Loop range based on the selected regions, i.e. the strt of the loop range is the start of the earliest region, and the end of the loop is the end of the latest region.
Set Punch: Same as above, for the Punch range
Add Single Range Marker: Same as above, for the Edit range
Add Range Marker Per Region: For each selected region, creates its own Edit range based on the boundaries of each region
Set Range Selection: Creates a range selection based on the boundaries of the selected regions
Fades
[] Fade In: Activates/desactivates the Fade In at the start of the region
[] Fade Out: Same as above, for the Fade out at the end of the region
[] Fades: Shortcut to activate/desactivate both the fade in and fade out
Duplicate
Duplicate: Creates a copy of the selected region(s) and happend it to the original
Multi-Duplicate…: Shows the Duplicate dialog, allowing to create multiple copies, or a not-integer number of copies (the last one will then be truncated)
Fill Track: Creates duplicates until it fills the session, i.e. reaches the End marker of the session. The last duplicate may be truncated to fit in
Export…: Shows the Export dialog, with all parameters set to export only the selected region(s)
Bounce (without processing): Creates a bounce, i.e. a version of the region with all the edits (boundaries, enveloppe), as a new region in the Editor List, without any of the effects of the mixer strip
Bounce (with processing): Same as above, with the effects of the mixer strip
Remove: Deletes the region from the edit (no file is harmed in the process, and the region stays in the Editor for later use)