irule sequence of events

CLIENT_ACCEPTED fires immediately after the TCP handshake happens (for TCP obviously), HTTP_REQUEST fires when each GET/PUT/HEAD/POST/etc occurs, but after CLIENT_ACCEPTED. For things that only need to happen once in a connection (IP/TCP comparisons or logging) it makes sense to do that in CLIENT_ACCEPTED so they aren't evaluated for each http request within a connection. If you need that data for use within the http events, you can always set a variable if the context changes.