If an effect would cause you to gain life past your maximum life total, you won’t gain any life above your maximum life total. For example, say your life total is 8 and your maximum life total is 10. You cast Azorius Herald, which has “When this creature enters, you gain 4 life.” You’ll gain 2 life, putting your life total back up to its maximum of 10.

Maximum life total means just that. After resolving You Compleat Me, you can’t have more than 10 life. Ever. No matter what.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.