The different of WCA,WCB and WCC is made of different steel grade.
C is better than B, B better than A. all there had different temperature and strength rating.
As for valve body selection (according to ISA)
"Control valve body materials shall meet, or exceed, the requirements of the application. Body and
trim materials shall meet the temperature requirements. NACE MR0175, “Sulfide Stress Cracking
Resistant Metallic Materials for Oil Field Equipment” shall be applied for all materials in sour fluid
services."
WCC & WCB both can be use for same application as far as material is concerned. But WCC has
more strength compare to WCB, for eg for similar temperature and pressure applications WCC
grade cardon body valve can be used with lesser pound rating as compare to WCB grade carbon
body.
The guiding document for control valve body selection is the Piping Material Specification(PMS).
From the PMS, one can pick the valve body material for the particular piping class corresponding to
the service in which the valve is installed.
If the PMS specifies WCB as the body material, WCC can also be selected instead, as WCC is
considered superior to WCB.
Other body materials like SS316, Inconel 825 though available and generally suitable for a service
on which WCB is specified, are to be selected only if the PMS demands it due to the associated cost
impact