Since Indigo supports Direct messaging I don’t really need to link the EZIO4X2 to the PLM as responder and the PLM to the EZIO4X2 as controller?
As far as Indigo controlling the relays, there is no difference as far as reliability is concerned. Both Group commands and Direct commands will be ACKed and retried as needed. Direct commands are easier to use since no link records are used or required.
All Smarthome device to device control is done with Group commands. An individual Smarthome device does not issue Direct commands to control another device. This allows multiple responders each with different Bright level and Ramp rate responses as that information is stored in the responder link records.
When using automation software/firmware Direct commands offer an additional option beyond Group commands. Also some command functions can only be issued with Direct commands.
The SHN Utility EZIOxx tab displays the Configuration Register. The EZIOxx tab | Input Settings and Monitor tab displays the Input Group numbers. Since the EZIOxx will not have links I suggest Factory Resetting the EZIOxx whick will clear all the links and reset all the Input Group numbers to 00. Indigo will set the Configuration Register with the Broadcast Status Chg option.