which device will be slave/client and which one will be master/server

hi
I Have one basic doubt,I was reffering the specification of MODBUS TCP,
I got confused.
According to my understanding, the controlling device i.e the RTU(Real
Time Unit) will be the slave and the one who request(poll) for the data
is the master.
But in case of client/server concept the one who serves the
request(RTU) will be the server and one poll for the data is the
client.

Please clarify my doubt
Thanks in advance
Prasi

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