C = Classic series
M = Military series
The only M head that is compatible with the C series is the M2 head.
To my knowledge the C series can include the D2/D3 (discontinued), C2/C3, G2/G3, G2Z, Z2, Z3, 6P, 9P, 6PD and their LED counterparts (usually an L after the name, such as G2L, Z2L, G2ZL, and so on.) Usually these have interchangable lamp assemblies (P6/9 series or LED heads), tailcaps, and so forth. The KT1 or KT2 turbo head assemblies can be used with this series for "throw."
Then there is Surefire's M series (millenieum??.) The M2 is a beefed up C2 (more o-rings and different design and higher cost) while the M1 is the infared light, the M3/M3T/M4/M6 are pretty much in their own league in terms of parts interchangability. This can also refer to the weapons light series which is an entire novel within itself due to SF's vast array of weapon lights. The X series is their pistol light series.
There is also the L series which refers to their "Digital Lumamax Series" L1, L2, L4, L5 (M2 body/tailcap with KL5 head), L6 are in this category.
The E series or executive series are a thinner more EDC type light. E1e, E2e, E1L, E2L, E2DL, E2D pretty much fall into this category.
I hope this sheds some light on your questions.