Like most abilities on creature cards, Glimpse, the Unthinkable’s abilities only apply while it’s on the battlefield. This means that if Glimpse, the Unthinkable isn’t on the battlefield, players are free to choose its name with Meddling Mage or choose Glimpse, the Unthinkable in a player’s hand with Thoughtseize.

Glimpse, the Unthinkable’s second ability prevents it from being chosen for spells and abilities that specifically ask a player to “choose” or make a “choice” among creatures or other permanents, such as found on cards like Call to the Void, Divine Reckoning, and Vesuvan Shapeshifter.

Glimpse, the Unthinkable’s second ability doesn’t prevent it from being chosen as part of a game rule, such as choosing creatures to attack or block with.

Glimpse, the Unthinkable’s third ability prevents players from choosing the name Glimpse, the Unthinkable for spells and abilities that require choosing a card name, but it doesn’t stop players from choosing the name Glimpse the Unthinkable. (This might be tough to explain out loud.)

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.