The undo/redo feature works on your database. This means it will only affect actions which change the database. We cannot give you a detailed list of what affects the database (DB) and what does not - just a few examples:
changing tree or sequence data affects DB (you've guessed that!)
changing the cursor position in ARB_EDIT4 does not affect DB
changing dialogs does _sometimes_ affect DB (this depends on whether the value is save in properties or in the DB).
Note that there is only _one_ undo/redo queue for all applications together, cause they all work on the same DB! If you e.g. change a tree in ARB and then change a sequence in ARB_EDIT4 you cannot undo your tree change by clicking UNDO in ARB, which would on the contrary undo your sequence change because it was your last DB-change.
After undo-ing something be careful: If you change anything (that affects the DB) your current redo-list will be deleted - this means you can no longer redo your undos!