Does commander damage from a commander that changed zones reset to zero?

In the Commander format, a player loses the game if they have been dealt 21 or more combat damage by a single commander over the course of the game (rule 903.10). This damage is tracked cumulatively — but only for a specific object instance of that commander card.

When a commander changes zones (e.g., it dies, is exiled, or is bounced to hand) and then returns to the battlefield, it becomes a new object with no memory of its previous existence (CR 400.7). Because the old object is gone, the combat damage it dealt is no longer attributed to the new object on the battlefield. The damage counter resets to zero for the new instance.

This is an important strategic consideration: if your commander is about to deal lethal commander damage, your opponent may deliberately remove it to the command zone or another zone to reset the clock. Conversely, an aggressive player wants to keep their commander on the battlefield continuously to accumulate damage.

Concrete example: Your Kaalia of the Vast has dealt 18 combat damage to an opponent. Your opponent casts Swords to Plowshares on Kaalia; you redirect her to the command zone. When you recast and replay Kaalia, she is a new object — the damage counter resets to 0, and she must deal another 21 damage to threaten that player with commander damage loss.

Note that commander damage from different commanders is always tracked separately, and each commander has its own independent 21-damage threshold per opponent (CR 903.10).