WWID - Where it come from?

Dear All,

I have been wondering a question for a long time that where WWID come from? (assigned by OS, storage system...).Based on my experience, WWID is assigned by Storage system but I do not know how it is formed (by WWNN or WWPN...). I have read many documents about this, however there is no document tell me details about WWID.

Could you help me have details about this?

Thank you in advance.