Q: When you use Suppressive Fire, do you place the template and determine who can participate (thus changing the size of the template), or do you declare the size of the template and then place it legally based on who is participating?
A: Declare the size of the template, then place it legally.
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Q: When a jack with a shield ends it's activation completely in a Suppressing Fire template, from where does the damage originate?
A: Damage from Suppressing Fire does not come from any arc, so effects that modify ARM against damage dealt to a particular arc do not apply.