In this rulebook I'm looking at, I see a whole set of rules that only apply if I choose to say, "We're playing with this stuff right here." Because it's optional, or rather, a whole subroutine of the rules that's marked as optional. We play either with it switched on or with it switche off.
A customizable aspect of System which allows participants to allow it to be present or absent during play, often for the whole of that particular group's play. A Dial with two settings (on/off). Also called a toggle. The term was first presented in Champions Millennium.