What goes into the undo list are undoable actions. Clearly you can't undo a Save or any other action which affects the hard disk but not the document. You can't use Undo to undo actions which affect Nisus globally, such as changes made to its Preferences, or which merely affect the display of a window, such as whether return-characters are shown. And of course you can't undo anything at all having to do with a document once that document's window has been closed, as we already know.
Global changes triggered by single commands count as single items in the undo list.
Running a macro is a single action, which I think is very nice, since if the macro doesn't do what you wanted the document is restored by a single Undo. The most confusing sorts of action, I find, are typing actions. This single Typing action does not end until either you move the insertion point, or you give some other menu command. This is true even if the first key struck in the Typing action is the delete-key. Throughout this section we have spoken of what appears in the undo list as a sequence of stages of the document.