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It is also important since you (As the creatures controller) decide the order of events during the upkeep.
Example: You have a creature with 5 life and 0 damage and Regeneration 2. During the upkeep, you have 5 rot tokens on that creature. Since you, the creatures controller decides the order of events, the most "Profitable" out come is as follows; Take 2 rot damage, regenerate 2, take the remaining 3 rot damage.
It also worth noting that conditions "Stack".
Also this.
VOLTARIC SHIELD
Each Upkeep Phase, the Wizard
may pay 2 mana to activate his
Voltaric Shield (flip the shield marker
to the ON side). The first time the
Wizard is attacked and dealt damage
this round, if the shield is on, flip the
shield marker to the off side, and
cancel up to 3 points of damage. If the
shield marker is still on at the end of
the round, flip it to the off side.