GATT defines the way that two Bluetooth Low Energy devices transfer data back and forth. Uses the generic data protocol called the Attribute Protocol (ATT).

GATT server & client

The peripheral is known as the GATT Server. It holds the ATT lookup data and service and characteristic definitions

The phone/tablet is the GATT Client which sends requests to this server.

All transactions are started by the GATT Client (the peripheral), which receives a response from the GATT Server.