Recently proposed to do dot, use the android bluetooth communication (BLE), baidu various examples also look not very understand, used to ask you brother

Now have access to the BluetoothGatt, then through BluetoothGatt access BluetoothGattService, found getServices () method can access to multiple service, any decision which one should be used?
Continue to get down BluetoothGattCharacteristic, found a service also have multiple BluetoothGattCharacteristic, should use which one? Should be how to determine which is the read and write data to use?