Editing : Editing Basics : MIDI Clips and MIDI Data : Nondestructive MIDI Editing

Nondestructive MIDI Editing
While editing audio clips is usually nondestructive, this is generally not the case for MIDI clips. For instance, if a MIDI clip resides in just one track at a single location, editing for that clip is destructive. This means that altering the pitch, duration, or placement of notes in Notes view permanently alters the clip.
However, if the Mirror MIDI Editing option is disabled, when editing a MIDI clip that appears elsewhere in the session (either on the same track at another location or in a different playlist, or in another track), editing is nondestructive and creates a new auto-created clip. To go back to the previous material, drag the original clip from the Clip List, or return to a previously saved playlist.
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One way to safely return to a track’s previous state is with playlists. Before you edit notes, trim clips, or rearrange the order of clips, make a duplicate of the track’s existing playlist and then edit the duplicate (see Playlists).
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To apply edits to all instances of a MIDI clip, enable Mirrored MIDI Editing mode (see Mirrored MIDI Editing).