Yes, enabling ILS also enables one more thing automatically, which is Remote Cluster View. Go to CUCM > Advanced Features > Cluster View and you'll see the opposite side CUCM cluster Publisher defined there. This allows, or forces really, the CUCM clusters to ask each other if the user belongs to the other cluster. In order for CUCM to know which cluster the user belongs to, the Home Cluster checkbox needs to be checked in one and only one of the two clusters.