What is the meaning of bus number in PCI Express?

Question

Iron

I don’t understand what is the meaning of the term "bus" when applied to pci express, because this is a star topology (there is no common bus).
Or is it atamism? And where can I read about the rules for assigning a bus number (I didn’t find it in the specification, maybe I didn’t read something)? I don't understand the logic behind this hierarchy.
Thank you.

Answer 1

Devices with the same bus number are connected directly to the same bridge (or root complex), and devices with different ones are connected to different ones.
Bus numbers are issued when setting up BIOS or OS bridges.