when an objective is completed, you use
"1" objstatus "done"
to tick off the objective in the briefing. in addition to this, create objective variables, like obj1, obj2 etc. in the init file and set them all to false. once an objective is completed, make the corresponding variable true, i.e.
"1" objstatus "done"; obj1 = true
and in your evac area trigger, player present, condition field
this and obj1 and obj2 and obj3 etc