As per my knowledge, Diameter is a peer to peer protocol and based on the messages it is decided which one is a client and which one is server. There is a convention if a node send request it is considered as client and the node which responds to request, is considered as server.
For example: In LTE network MME and HSS both the nodes work as Diameter nodes.
For few messages, MME is Diameter Client and for few other messages HSS is client.
