Rule 510.1c

A blocked creature assigns its combat damage to the creatures blocking it. If no creatures are currently blocking it (if, for example, they were destroyed or removed from combat), it assigns no combat damage. If exactly one creature is blocking it, it assigns all its combat damage to that creature. If two or more creatures are blocking it, it assigns its combat damage to those creatures according to the damage assignment order announced for it. This may allow the blocked creature to divide its combat damage. However, it can't assign combat damage to a creature that's blocking it unless each creature that precedes that blocking creature in its order is assigned lethal damage. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that's being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that's actually dealt. An amount of damage that's greater than a creature's lethal damage may be assigned to it. Example: The damage assignment order of an attacking Craw Wurm (a 6/4 creature) is Wall of Wood (a 0/3 creature) then Eager Cadet (a 1/1 creature). Craw Wurm can assign 3 damage to the Wall and 3 damage to the Cadet, 4 damage to the Wall and 2 damage to the Cadet, 5 damage to the Wall and 1 damage to the Cadet, or 6 damage to the Wall. Example: The damage assignment order of an attacking Craw Wurm (a 6/4 creature) is Wall of Wood (a 0/3 creature) then Eager Cadet (a 1/1 creature). During the declare blockers step, the defending player casts Giant Growth targeting Wall of Wood, which gives it +3/+3 until end of turn. Craw Wurm must assign its 6 damage to the Wall. Example: The damage assignment order of an attacking Craw Wurm (a 6/4 creature) is Wall of Wood (a 0/3 creature) then Eager Cadet (a 1/1 creature). During the declare blockers step, the defending player casts Mending Hands targeting Wall of Wood, which prevents the next 4 damage that would be dealt to it. Craw Wurm can assign 3 damage to the Wall and 3 damage to the Cadet, 4 damage to the Wall and 2 damage to the Cadet, 5 damage to the Wall and 1 damage to the Cadet, or 6 damage to the Wall. Example: The damage assignment order of an attacking Enormous Baloth (a 7/7 creature) is Trained Armodon (a 3/3 creature) that already has 2 damage marked on it, then Foriysian Brigade (a 2/4 creature that can block an additional creature), then Silverback Ape (a 5/5 creature). The damage assignment order of an attacking Durkwood Boars (a 4/4 creature) is the same Foriysian Brigade, then Goblin Piker (a 2/1 creature). Among other possibilities, the active player may have the Baloth assign 1 damage to the Armodon, 1 damage to the Brigade, and 5 damage to the Ape, and have the Boars assign 3 damage to the Brigade and 1 damage to the Piker.

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