wrote:

Unusable fuel is that
fuel that won't drain from the tank with the aircraft in the most
critical attitude for flight; with some of the Cessnas, that's nose-
low. Some people think it's just the fuel in the lines ans strainer;
it's not. The unuseable fuel is the difference between the total fuel
posted on the wing next to the filler) and the capacity noted on the
fuel selector. You can also find it in the TCDS on the FAA website.

You can also find it in the POH as you've learnt in your primary training.