SCSA Slot Question

OK, so completely new to the SCSA aspect of competitive shooting as my son has only done SASP matches up until this past year and has earned B classification so far in 3 divisions as a 13 year old. Now, my question is, how does slotting work once he competes this month in our Area match? Is it solely based on division? Class? Category(age group)? Mix of all 3? I know the slots are not guaranteed, but just looking to see what all goes into determining the chance for slotting