You can use "-d" option for sysctl. It provides short description for each system control like
dev.pcib.3.subbus=Subordinate bus number
dev.pcib.3.secbus=Secondary bus number
dev.pcib.3.pribus=Primary bus number
dev.pcib.3.domain=Domain number
In particular case this is statistics information about buses attached to PCI bridge (aka pcib). So it's doesn't map to PCI ID.
Sysctl is just routine which gathering tunable / read-only variables over kernel modules. As result, it doesn't know meaning of each variable. Developer of particular functionality may describe meaning of sysctl variables, but I think it's rare case.
If you're looking for PCI information, it's worth to use "pciconf -l -v" and "devinfo".