on qemu-manpage (https://www.qemu.org/docs/master/system/qemu-manpage.html)

on qemu-wikipage (https://wiki.qemu.org/Documentation/Networking)

-netdev TYPE,id=NAME,...

The id option gives the name by which the virtual network device and the network backend are associated with each other. If you want multiple virtual network devices inside the guest they each need their own network backend. The name is used to distinguish backends from each other and must be used even when only one backend is specified.

Maybe the content on wiki page is more suitable, please have a check, Thanks. Regards