If i'm on a single tank rec dive, & I sling a tank that I haven't planed to use for extending bottom time or to accelerate deco, but I have planed to forget about, (except to charge the reg occasionally), unless faced with some sort of emergency OOG situation, what is it?
A stage bottle, regardless of weather you use it the bottle is still part of a "staged" dive plan it is there for a contingency.