GATT

Data model of BLE connections: services, characteristics and descriptors exchanged over the Attribute Protocol.

protocol also known as Generic Attribute Profile, ATT, Attribute Protocol

Knowledge about GATT

Android GATT: queue every operation and clean up after status 133procedure · unverified · Android's BluetoothGatt runs one operation at a time; issuing the next before the callback silently drops it. Serialize operations in a queue, and close the BluetoothGatt object on errors such as status 133.
BLE for IoT devices: roles, GATT and the numbers that matterguide · unverified · Orientation for building BLE IoT devices: GAP roles, the GATT data model, advertising vs connections, and the limits you hit first.
BLE notifications are cut to 20 bytes until you negotiate the MTUwarning · unverified · The default ATT MTU of 23 bytes limits each notification to 20 bytes. Negotiate a larger MTU and enable Data Length Extension to send up to 244 bytes per packet.