rules for interface naming for on-board cards

Hi, I am trying to understand the way interface names are given. So one way is hard coding in net_device->name. If the name contains a %d format string, the first available device name with the given base is used; assigned numbers start at zero. On my base machine, I have 1 NIC card which is onboard, rest I can remove and plug. For the onboard card I get the interfaces as em1-4, while all the other cards I get names as p1p2, p3p4 etc. Does Udev know the difference. How are these names given, definitely there must be some configuration which is used to distinguish on-board card vs removable. Thanks.