That's a good question. What is an activation? Is it a "callout" enmasse? Does it mean "all or nothing"? Is it a fifty year old idea of passing traffic by the basket-full? Or, could it be different in today's world?
It could be that the idea of "all or nothing" comes from the call-out tree as used by emergency communications units. That is where the Radio Officer calls one to three key persons who then call 3 to 5 people, who then call three to five people, etc. That process works well; but it can imply a full scale callout to those unaware that scheduling of shifts begins as soon as a need is apparent.