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If an ability would produce one or more mana of an undefined type, it produces no mana instead.

Some abilities produce mana based on the type of mana another permanent or permanents “could produce.” The type of mana a permanent could produce at any time includes any type of mana that an ability of that permanent would produce if the ability were to resolve at that time, taking into account any applicable replacement effects in any possible order. Ignore whether any costs of the ability could or could not be paid. If that permanent wouldn’t produce any mana under these conditions, or no type of mana can be defined this way, there’s no type of mana it could produce.

To “tap [a permanent] for mana” is to activate a mana ability of that permanent that includes the {T} symbol in its activation cost. See rule 605, “Mana Abilities.”

An ability that triggers whenever a permanent “is tapped for mana” or is tapped for mana of a specified type triggers whenever such a mana ability resolves and produces mana or the specified type of mana.

A replacement effect that applies if a permanent “is tapped for mana” or tapped for mana of a specific type and/or amount modifies the mana production event while such an ability is resolving and producing mana or the specified type and/or amount of mana.

If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3i.

The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

The untap symbol is {Q}. The untap symbol in an activation cost means “Untap this permanent.” A permanent that’s already untapped can’t be untapped again to pay the cost. A creature’s activated ability with the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since their most recent turn began. See rule 302.6.

When you cast Divination from your hand, you’re losing a card (Divination itself), but after it resolves, you’ve gained two cards in your hand. Overall, you’ve gained a card. Opponents haven’t gained any cards, so you’ve gained card advantage. Conversely, when you cast Prosperity for X=4, you lose a card (Prosperity itself), but you gain four cards, for a net gain of three. However, even with just one opponent, that player will draw four cards, so that ends up being a total net loss of one card for you, which means Prosperity with X=4 was not card advantage.

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!).

Abilities that cause you to gain card advantage won’t cause Toofer’s ability to trigger, even if those abilities trigger from casting spells. Similarly, card advantage from other sources, like combats where opponents lose more creatures than you do, won’t cause Toofer’s ability to trigger.

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.

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.

Card advantage is determined based on what actually happens in game. For example, Annihilate will be card advantage if it resolves normally targeting a creature an opponent controls—you lose one card from your hand but you also draw a card, for a net gain of zero cards, and your opponent loses one card, so you’re up one card relative to them. However, if the target creature has indestructible, you and your opponent both have a net gain of zero cards, so that’s not card advantage.