The first system IPL

Figure 1 shows the multisystem environment after SYSA is initialized.

Figure 1. One System in the Multisystem Sysplex

When SYSA is initialized and joins the sysplex and the global resource serialization complex:

XCF coupling services are available on SYSA, but signaling services are not available between any of the systems.
A primary sysplex couple data set is used for SYSA (not shown).
MVS™ component exploiters can exploit XCF coupling services on SYSA but must use non-XCF connections to communicate with other systems.
Multisystem applications can exploit XCF coupling services on SYSA.

Consider this mixed complex a temporary configuration during migration until the sysplex matches the complex.