What is the notion of sub-IP?
It defines the Subordinate IP when there is a hierarchy or dependencies to the parent IP.

https://help.sap.com/doc/saphelp_crm70/7.0.0.18/en-US/8e/206a62137b4df19172bb63031f7150/frameset.htm

For example some libraries may be a sub-IP to a complex ASIC with a parent IP.