<P> When multiple users can edit the same document simultaneously, a multi-user undo is needed . Global multi-user undo reverts the latest action made to the document, regardless of who performed the edit . Local multi-user undo only reverts actions done by the local user, which requires a non-linear undo implementation . </P> <P> Where undo can be used to backtrack through multiple edits, the redo command goes forward through the action history . Making a new edit usually clears the redo list . If a branching redo model is used, the new edit branches the action history . </P> <P> The number of previous actions that can be undone varies by program, version, and hardware or software capabilities . For example, the default undo / redo stack size in Adobe Photoshop is 20 but can be changed by the user . As another example, earlier versions of Microsoft Paint only allowed up to three edits to be undone; the version introduced in Windows 7 increased this limit to 50 . </P> <P> Simplistic, single - edit undo features sometimes do away with "redo" by treating the undo command itself as an action that can be undone . This is known as the flip undo model, because the user can flip between two program states using the undo command . This was the standard model prior to the widespread adoption of multiple - level undo in the early 1990s . </P>

What is the function of undo and redo