A frame is a skeleton of terminals, where terminal values are used to represent a particular entity. "In principle, we could use frames without attaching their terminals to anything. Normally, though, the terminals come with other agents already attached -- and these are what we called 'default assignments' when we first talked about level-bands."
Frames define expectations, based on past experience, to faciliate processing the present. But frames won't make perfect predictions, so we must learn to adapt them.
NB: How do you detect imperfect predictions? By detecting a difference between an activated frame and an agency representing the present state of affairs!