Note the difference please.
Public Actions: Take place on Saturdays and are generally "get together" play sessions on the public server on the map that is on rotation.
Open Events: Take place on the last Friday of a month and are mid-week op style missions that are open to the public.
I am pretty sure I know this because this was created in a joint effort of th
e media team and the recruitment team a about a year ago. We officially changed the name for this to
3CB Open Event since it caused a lot of confusion in combination with the term Public Action. As seen here, it is pretty easy to make the mistake mentioned. This is why I hope that the term will be used when we talk about this to prevent these mistakes.

As mentioned, the
open events run on Zeus templates and generally speaking they always only include roles, equipment and vehicles that the mission designers wants in the mission itself. This is not the case for Public Actions which are done the way Reddog described them.
Monitoring player numbers could be a bit tricky since they could also be misleading. It is possible that the player numbers do not change by much, but that the origin of the players on the server could change. So this could mean that we maybe get more 3CB members playing on the server but we loose public players who are not affiliated with 3CB. The problem that I see with that is that I think that the longer a player is part of 3CB, the less likely he is to play on the public server outside of public actions or open events.I think that this is a somewhat natural development since the player has 3-4 opportunities during the week to play ArmA 3 in a way that made him become a member of 3CB and stay inside the unit. So he can get his his ArmA 3 fix there and it is less likely that he will play on the server.
The worst case scenario to me is that we have a public server that is only populated by our members.