I have one PC on which we installed kepware software (MODBUS-TCP client). Now we connect one Gateway device (MODBUS-TCP server). We have connected 10 devices to this Gateway device through a wired connection. Now as per my understanding, if I want to configure client, then one way is just allocate 10 different register address for each device parameter like 10000 for device-1 (voltage), 10001 for device-2 (voltage) and so on. But if all devices are in the same category (Means they are providing their voltage), then I think this is the wrong way for development. So can we do like allocating each device as one unique number (we can called it a device identifier), and get their parameter value of the device through register address 10000? If its possible, then can you explain to me how to achieve this or do you have any reference document?

I suppose you have one Modbus TCP gateway and 10 serial Modbus RTU devices? If you use a Modbus TCP to RTU gateway like the:

https://www.moxa.com/en/products/industrial-edge-connectivity/protocol-gateways/modbus-tcp-gateways/mgate-mb3180-mb3280-mb3480-series

This gateway in it's most basic configuration provides all routing between TCP and RTU.

Every Modbus RTU slave should have a unique ID. In Kepware you can access all these devices independently by using the IP address followed by the node name: '192.168.1.10.x' Search for 'Bridge' in the Kepware manual. Now you can use the same registry addresses for every node.

If this is not a usable answer to your question please provide detailed information about all the components you are using (Supplier, model, ...)