I am reading Fischer, Michael J., Nancy A. Lynch, and Michael S. Paterson. "Impossibility of distributed consensus with one faulty process." Journal of the ACM (JACM) 32.2 (1985): 374-382, available here, the paper in which the well known FLP Theorem was proved.
I am having trouble understanding the notion of an "applicable event" used in this paper. Here's a quote from the paper (Section 2):
Since processes are deterministic, the step is completely determined by the pair $e = (p,m)$, which we call an event. (This "event" should be thought of as the receipt of $m$ by $p$.) $e(C)$ denotes the resulting configuration, and we say that $e$ can be applied to $C$.
I can't understand this definition. When is an event applicable? When is it not applicable?