F - Flat rate
Flat rate - The amount of interest on a loan per year as a direct percentage of the amount borrowed. This is easier to calculate and understand than the APR.
However, don't try to compare the two, because a flat rate will make a loan appear far cheaper than it really is, since it ignores any additional fees and charges.
The best way to compare loans is to look at the APR - don't get fobbed off with just the flat rate. (See APR and Finance.)