What is an LUID

Good Day
Judging by the screenshot below an LUID is a Logical Unit ID.
Is that correct? (Q1)
I see various references to LUID in this config guide and the forums.
It seems it is some sort of handle assigned by an AP for any session with a SM.
Would anyone be able to provide a definition? (Q2)
Doe they in fact apply to APs only not SMs (Q3)

Much appreciate any replies

I think the AP is LUID 1 but the LUID is how the AP references the SM.