I think he meant the difference between PPGA and FC-PGA, both of which fit into the same socket called Socket 370.
Basically, PPGA (Plastic Pin Grid Array) came first, these are your slower Celerons and are also referred to as Socket 370 chips.
Then came FC-PGA or flip chip pin grid array which is similar, but the chip is flipped and the big heat spreading slug is gone. The FC-PGA pins look the same as PPGA chips, but the pinout is different.
As such, the BIOS must support FC-PGA if an FC-PGA chip is used.
jeremy806