What is the purpose of "shortId" field in Reality configuration?

[mansaliacom]

It seems to be good practice to give every client a unique shortId, but what does it actually do?
Most references I found to it in the source code were boilerplate code.
The only other one was this:

fmt.Printf("REALITY remoteAddr: %v\ths.c.ClientShortId: %v\n", remoteAddr, hs.c.ClientShortId)

...which is just for debugging purposes.