A non-client is an IBGP peer that functions as neither an RR nor a client.

 Non-Client Definition in IBGP

In an IBGP setup with a Route Reflector (RR), a non-client is an IBGP peer that is neither the RR itself nor its client.

Non-clients must establish fully meshed IBGP connections with all other non-clients and the RR because IBGP rules prohibit route propagation between non-clients without a direct connection.

 HCIP-Datacom-Core Reference

The behavior of non-clients in an RR topology is clearly outlined in the BGP implementation chapters​​.