When you're getting into a game as old as 40k, there are going to be some elements that are holdovers from previous editions. I'm not 100% sure the term 'force org(anization) chart' even appears in this edition.
'Force organization chart' was what detachments used to be called, way back when you could only have one, lol. Eventually the name changed (and 'force org slot/role' became 'battlefield role').
'No force org slot' means that it doesn't occupy a position in a detachment chart. In our army, that's mostly going to be used in the Court of the Archon models.
Unfortunately, there are some parts of the hobby that just have to be learned with experience. (I still have dudes at my local game store who talk about whether or not a model gets 'spitties.')
EDIT: Also, moving this to Rules Questions.