Auto: Groups the actions of a command, making them all reversible by a single undo.
Begin/End: This option groups actions together driven by the user. I use this in lisp.
Mark/Back: Mark places a book mark in the undo log. At a later time you can chose to undo back to the last Mark you placed in the log. If there are no marks, undo back will take you back to when you first opened the drawing. You will get an alert telling you so. I like this for the times I wished I nevere open a drawing in the first place.