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.
State OES headquarters has an alert code of colors to signify different states of "alertness" or "activation" from an operations standpoint. Green means situation normal; yellow means a watch condition exists for the radio room and/or the personnel; orange means staffed with minimum duty personnel; red means staffed for a major incident and personnel should check in on a special phone line with their availability status. Our callout is always selective; that is, the talents, skills and experiences needed vary from incident to incident. We select from our roster database those most likely to fit the need, and then proceed to setup schedules from the responses. Only in a major disaster does a limited group report to the HQ facility without callout. At State OES it can range from "we need help over here" to a written request to the program coordinator or the chief radio officer.
How it works elsewhere depends on who has the authority to activate, and the degree of trust that exists between the people involved. In new units it can be very formal. In units where the people have worked closely, it can be very informal. To government emergency management agency staff, activation of an EOC is turning on the lights and calling in the people. For RACES, it will vary with the jurisdiction and the people involved.