in the same way, other nations designate their fighters it would make sense: J-10A, B, C ... and so the standard Block 1 & 2 JF-17s could be called JF-17A, B is the twin seater and the modernised single seater then would be the C and its trainer the D.
Actually I don't know if this is that way, but at least it would make sense.
A few years ago when Block-II had shown up, we learned that Block-I will be upgraded to Block-II standard and the Block-II will be PAF's baseline model of JF-17.
If this is true, then:
Block II = JF-17A
Block-II twin-seat = JF-17B
Block-III = JF-17C
Block-III twin-seat = JF-17D
F-16s too uses A/B/C/D/E nomenclature.