Complex sentence: When a read to a specified BDF combination vendor ID register succeeds, the BIOS or OS knows it exists, and can then program the memory mapped and I/O port addresses the function will respond to into the devices' BAR configuration register.
Split into simpler sentences: When a read to a specified BDF combination vendor ID register succeeds, the BIOS or OS knows it exists. At this point BIOS or OS program the memory mapped and I/O port addresses the function will respond to into the devices' BAR configuration register.