request_mem_region

Hello As I understand ioremap() maps and returns virtual address for a given physical(bus) address associated with my PCI card. That is cool and clear. What is the purpose of request_mem_region()? They both take physical address as a parameter, but do not return any virt addresses. 1. What do they allocate/reserve and where? 2. Should I call reques_mem_region() before doing ioremap() or not? thanks