Limits to undo

Sometimes, I can undo changes even though I just :wrote the file to disk. The undo simply causes the file to change to an earlier state in time, which causes it to differ from the version written to disk. Hence, there is a [+] sign in the status line. In fact, undo seems to be able to undo changes even though there may have been multiple :writes to file during those changes.

Other times, it clearly can't go back to the original state of the file. For example, if one were editing a file "memberNumber.txt" and forgot that it wasn't tmp.txt, one could do some elaborate composition of text and multiple saves. After realizing that one was clobbering the content of memberNumber.txt rather than tmp.txt, one (the royal "one") can attempt to undo ad nauseum to get back the original content of memberNumber.txt. However, there is a point which it no longer undo's, and it is clearly not when the file has gone back to the state of its original content.

What determines whether undo can proceed backward in time beyond a certain point? The help pages don't seem to explain the above limit.

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