What we consider Spam is the obvious stuff - adverts for kitchens, beauty products, fake documents - stuff which has no relevance to the forums and which was clearly posted purely for advertising. The kind of people who post that have no intention of ever being contributing forum members.
Self-promotion comes more into the category of “inappropriate”, along with fluff and most other things.
Although the notes on the off-topic flag say it should be used for posts which ought to be moved to a new thread (asking a new question in an existing thread, for instance), it can be used for any off-topic post that’s simply adding “noise” to a thread.
But all flags will be reviewed by a moderator anyway, so don’t waste time worrying about which category is the right one; just take your best guess and press the button.