When you go on it, you'll do the 3 miler and gym test 1 (bleep test, sit-ups, press-ups, pull-ups). If you don't do well on this day (RMAC royal marine acquaint day), i.e. get the min scores (or very close to them) that'll pass a PRMC, then you won't be put forward to go on a PRMC.
That's not what my AFCO told me, he said it will give you an idea if you're ready or not but will not affect whether you can attend the PRMC. Though you most likely wouldn't want to go soon if you didn't do well enough.