"Level X: Advanced Tactics" contains tactical problems that will challenge a player rated Elo or USCF 1700 or higher if attempted without using any of the hints.
"Introduction to Tactics" introduces the student to the tactical tools of chess (forks, pins, etc.).
"Intermediate Tactics" presents tactical exercises that are significantly more difficult than those seen up to this point.
"Step Up in Tactics" presents exercises, both checkmates and tactical, which are somewhat more difficult than the material covered so far.