The Mechanical Comprehension section of the ASVAB practice test measures your understanding of basic mechanical principles and mechanisms. You may be asked why an intake valve on a pump opens when the piston goes down, or what direction friction is going when shown a diagram of a skier. The CAT-ASVAB has 16 questions in 20 minutes; the paper-and-pencil version has 25 questions in 19 minutes.
One ASVAB sub-measure, the Armed Forces Qualification Test (AFQT)--which is just a separate analysis of the same tests that focuses only on four of the subtests--is also population-referenced, but its score from 1-99 is a percentile score. This means that if you score a 73, you scored as well as or higher than 73% of test-takers in the national sample.

The Armed Forces Qualification Test (AFQT) is used by all of the Services to determine if an applicant is eligible for the military. Four of the ASVAB subtests are combined to form the AFQT. It measures general cognitive ability and is composed of verbal and mathematics subtests. AFQT scores are grouped into categories for reporting purposes. The table below shows the AFQT categories and the percentile score ranges corresponding to the categories. Applicants that score in AFQT category IIIA or higher may qualify for enlistment incentives.