netX 51 is used as a Host Controller in this case. A netX 51 based hardware like NXHX 51-ETM must be connected to any netX based companion chip which runs the PROFINEt or EtherCAT Slave LFW
I.e. in this setup 2 netX chips are used: One as host, the second as companion chip.
In most cases, netX 51 will not be used as a host controller. Anyway this example might be helpful for users to understand the usage of PNS V3 and ECS V4 stacks.