What's an "applicable event" in the context of the FLP paper?
An event $e$ is applicable in configuration $C$ if it could happen at configuration $C$. Since $e = (p,m)$ means that message $m$ arrives at processor $p$, $e$ is applicable at $C$ if the message queue at $C$ contains the message $m$ directed at $p$. If there is no message directed at $p$ with contents $m$ then $e$ is not applicable at $C$.