The fleet advert system is a simple mechanic that allows you to automatically accept people based on standings. This can be a very useful thing for bonus fleets, and pickup gangs, but at the same point its quite easy to end up advertising your fleet to far more people than you want. And if you have the auto accept on, its quite possible to suddenly find yourself scrammed by a hostile target wearing the purple fleet tag.
This is how I see the current contact standing system.
I work on the NBSI policy, which keeps things relatively simple.
Excelllent! Dark Blue
+10 standings. These are people I really trust, and would fly with at any time. (Usually my alts, and a few close in game friends) people that I would trust with my ship/pod.
Good! Light Blue
These are friends/NAP, you won't shoot them, they won't shoot you, you might even invite them to fleet sometimes, but only by invitiation.
Neutrals / Grey
Just people I want to keep an eye on in the logon watch list, or added to my contact list for easy communication. Too many people blue people that aren't really blue.
I use this to keep track on certain people or groups, so as identify their scouts/members quickly in local.
EVE-Uni are in this group, as are the Tuskers, and quite a few out of corp scouts for other gangs.
I don't actually use this tag on a permanent basis, its reserved for war targets, or groups that I wish to target on a given time frame.
So when I set up a fleet advert I use the following.
Corp [x] Hell you've got to trust someone.
Milita [ ] <-- no chance, this is a suicide button.
Alliance [ ] depends on the alliance, personally I prefer using the standings options.
Based on standings
Everyone that can't see this fleet advert will have to be invited personally, after all its not that difficult to x up.
Things to remember.
Fleets are based on the boss's personal standings, so they might not be quite your own view, especially when your flying with larger alliance, or hook up fleets. When flying in someone else's fleet I often keep the fleet window open just to see who's joining. Also remember that squad commanders can invite people into their squad, even against the standings of the boss.
Hopefully this way you won't end up like poor Russkinor, who recently found out to his cost that his boss had set up his fleet badly and was advertising it to all the wrong people...