Any device that wants to transfer data over Bluetooth can adopt a role based on the requirement. A device can act as a GATT Server which welcomes requests from the client, process data accordingly and returns back the value. GATT Server is responsible for processing and making data attribute available to the client.
GATT client, on the other hand, initiates the request to the server and receive the responses. A GATT client needs to perform a service discovery process in order to know the server's attributes. Once it is done identifying the services, it can read or write data attribute.
