A 400 pin 10 x 40 position connector form factor on virtex 7 FPGA is
used for both versions.
1. The HPC version is fully populated with all 400 pins present.
2. The LPC version is partially populated with 160 pins.
I know these FMC's are useful for logic expansion,
when LPC is used and HPC is used which one to decide among two and what are the factors decides them ?
They are electrically the same, so the only deciding factor is how many user pins you need for your application and whether or not you will have to interface to an LPC connector from your LPC/HPC connector.