The program keeps track of the edit buffers by treating them as separate items from the items in a bank. The voice edit buffer is called the VCED and the performance edit buffer is called the PCED in the sysex part of the TX81Z manual, so these are the labels I used to represent their types in the snapshot.
Voice edits always affect the VCED item in the snapshot, and performance edits always affect the PCED item. It's not possible to edit a bank or library item in-place, it has to be copied to one of the edit buffers first. The other items, like the program change table and the system setup, don't have edit buffers, so they are edited in-place.