My experience matches MichielR's (though my longhauls out of BRU are generally from terminal T (where they have all the flights to Africa)), which may be slightly different from terminal B (which, I think, has the other longhauls).

A distinction between business and economy with a separate line (as noted, headed by a document check) for priority.

I don't think that premium economy is with the priority boarding, but I'm not sure.