What is the last known information rule and how does it apply to a creature that leaves the battlefield?

When a permanent leaves the battlefield, it ceases to exist as a game object and normally loses all its characteristics. However, many abilities and rules need to reference what that permanent was — its power, toughness, mana value, abilities, etc. The last known information rule (CR 608.2h) solves this by instructing the game to use the object's last known characteristics from the zone it just left, at the moment it left.

This rule applies in several important situations: calculating damage or loss of life from a dying creature's triggered ability, determining a spell's effects that reference a creature that moved zones, checking whether a replacement effect applies to an object that has already moved, or figuring out a dead creature's power for something like a 'when this creature dies, you gain life equal to its toughness' trigger (CR 608.2h).

Crucially, last known information is used only when the rules or an effect specifically need to check characteristics of an object that is no longer in the relevant zone. If the object is still present (e.g., still on the battlefield), you use its current characteristics instead.

Concrete example: You control a 5/5 creature with the ability 'When this creature dies, you gain life equal to its power.' Your opponent casts a spell that gives it −3/−3 until end of turn, and it dies as a 2/2. The triggered ability uses last known information — the creature's power as it last existed on the battlefield was 2, so you gain 2 life, not 5.

Note that last known information does not help you determine what zone an object moved to, or track an object across zones for purposes of identity (that is handled by the 'same object' rules, CR 400.7). It is strictly about characteristics at the moment of departure.