Modbus buscoupler X20BC0087 + X20CS1030

It seem that in this constellation only flat stream is supported, so library AsFltGen is needed in the plc.

Serial communication is also possible with X20BC008U module, but in this case the frames are received on the bus coupler itself and then transported with the OPC UA protocol.
See link:

Why is the X20BC0087 used and not for instance powerlink X20BC0083 ?