The XX is just the register number for the remote device, the register numbers are there in the table and by the looks of it the value is held in two separate registers so you’d need some math to reconstitute them into a normal value reading. Modbus can be very cumbersome I admit.
It will be easier to integrate using Bacnet, that way you can use a statement like
10 VAR1 = 222AI3
Where 222 is the bacnet instance of the subnet device and AI3 is input three on the device. You will need to set the protocol of the subnet to Bacnet for this to work.
I use Bacnet and modbus in similar configurations. All works, just bacnet controllers are slower than controllers on modbus, and you can’t update TSTAT10 (T3OEM in my case) on Bacnet. Just my subjective opinion, because controllers performs different tasks and I haven’t done side by side test. I would use modbus, but i am lazy, so i use Bacnet.