The Modbus RTU gateway is a Modbus master, and can poll up to 50 slave devices. The remote device "read rule" mainly says what remote register to read and where to put the data. But you have the option of scaling data as it comes in. You also have the option of applying a default value if the read fails.

The remote device "write rule" allows setting up a "send on delta" to only write the register to the remote device when the local data has changed. You can also send periodically, with optional data scaling on the way out.