Various. Usually an offset into a struct. For example, ONAME nodes that refer to local variables use it to identify their stack frame position. ODOT, ODOTPTR, and OINDREG use it to indicate offset relative to their base address. ONAME nodes on the left side of an OKEY within an OSTRUCTLIT use it to store the named field's offset. OXCASE and OXFALL use it to validate the use of fallthrough. Possibly still more uses. If you find any, document them.