why does this need to be done? is it necessary everytime a head gasket is changed? or only if it needs doing? and how are you able to tell if it needs doing???

how can you tell if a head has been skimmed before? and what are the chances of an experienced company skimming the head incorrectly?

also what are the difference between the different head gaskets, standard, groupe a, and WRC (multilayer blablabla) and are there any others

thanks