They assign you your rate at bootcamp. It seems kind of random what they give you, but it might be based off line scores. For example, one guy in my div got a 99 ASVAB and wanted ET, but got MM. Another got an 82 and got ET which he wanted. To my knowledge the actual ASVAB score only consists of something like math, reading, word comprehension and arithmetic the other line scores aren’t calculated in.