To ensure that the Cisco Nexus 7000 series switches are the primary devices for LACP, the system priority needs to be configured with lower values indicating higher priority. In this case, setting N7K_A with a system priority of 100 and N7K_B with a system priority of 200 ensures that N7K_A has a higher priority and becomes the primary device for LACP. The system priority is used in the LACP system ID which determines the LACP priority; the lower the value, the higher the priority.
References: The configuration guidelines for LACP on Cisco devices can be found in the Cisco documentation, which explains the role of system priority in determining the primary device in an LACP setup1. Additionally, the Cisco Data Center Core Technologies study materials provide detailed information on configuring LACP and vPC domain parameters.