There was no designation for British aircraft (i.e. P for Pursuit, C for Cargo); only codes that can follow the names. In the case of Spitfire L.F.IXe, the "L.F." means light fighter, the "Mk" stands for Mark (meaning major variant), the "IX" is the variant of the design, and the "e" is the wing version (the IX had more than one wing version).