There is very little info out there, but what I dug out was
GPC = "General Purpose Core" - This is the primary core of your GPU
L2C = "L2 Cache" - This is the on core memory that your GPU core has access to. Running it at the same clockspeed as the core itself nets a large gain.
XBAR = "Memory Controler Crossbar" - This is the bus speed of the memory controller that talks to the core.
SYS = "System" - This is the rest of the card that isn't tied to another clock domain.
In my case, I discovered that core clock (no matter what voltage) above 1392 gives me blackscreens in The Witcher 3, so I currently limited all my clocks to 1392.5 and also set all those max values to it. Which interestingly is also what's in the reference BIOS of the GTX 970.
I think it is definitely the case that dropping the values there might yield you higher GPU clocks, since this is exactly what all those vendors who make factory OCed cards do! On my EVGA, some values there are as low as 1344, but then a higher GPC. So or so, I have all of them at 1392.5 now since currently I am not chasing after overclocks but want highest stability. (Amazingly my EVGA which craps out at 1418 in The Witcher needs only 1.125V at 1392 and is then stable....)
Bios modding and flashing until the EEPROM smokes!! Yay!!