Like Jay said. Please don't put out information that isn't true. PMD stands for "post mint damage". Pure and simple.
Not always. Chips or cracks or other damage happening after the die is in use cause die varieties also.
Wrong again. Chips and cracks are the result of a natural process in a die's life. They may be used as markers to help confirm a die variety, but die varieties don't get created after the die is in use. Die varieties are created during, and only during, the creation of the die. Die cracks,die chips, and die damage are considered die errors to some, but a die variety was on the die before it ever struck the first coin. As enamel7 said, please stop putting out false info. Even die clashes aren't considered varieties. Most considered them stages in a die's life.
Although some may argue, the '3 legged buffalo' is considered a variety by most, and it was created by repair of a used die that was removed for that purpose after having struck coins, and then put back into service after repair.
This is true. I don't consider it a die variety, but most do. The 1969-D No FG Lincoln, and the FG variety Kennedys were created this way. Also, don't forget the 1922 No D Lincoln. I just can't see paying that kind of money for an abraded die. Sorry for saying "only". Still I consider DDOs, DDRs, RPMs, RPDs, OMMs, and such die varieties, but that's just me. A lot of other folks feel differently. Don't forget the 2 feathers buffalo nickels either. That being said, cracks, chips, and post strike damage are not die varieties.