Halving Season’s abilities won’t change the number of tokens you create or counters you place, even if you’re placing counters on permanents your opponents control.

If a permanent enters the battlefield with counters on it, the effect causing the permanent to be given counters may specify which player puts those counters on it. If the effect doesn’t specify a player, the object’s controller puts those counters on it.

If two or more effects attempt to modify how many counters would be put onto a permanent you control, you choose the order to apply those effects, no matter who controls the sources of those effects.

If a player would create tokens or put counters on a permanent or player while their opponents control two or more Halving Seasons, apply the replacement effects one at a time, rounding down each time. For example, if a player would create 30 Treasure tokens while their opponents control three Halving Seasons, they would halve 30 to 15, halve 15 to 7, and then halve 7 to 3, and end up creating three Treasure tokens. Don’t spend them all on one spell!

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.