Hi Xinyu,
Please note that related events are handled in BleApp_Gatt ServerCallback ().
Please refer to "Bluetooth® Low Energy Application Developer’s Guide".
There is an important difference between sending Notifications and Indications: the latter can only be sent one at a time and the application must wait for the Client Confirmation (signaled by the gEvtHandleValueConfirmation_c Server event, or by a gEvtError_c event with gGattClientConfirmationTimeout_c error code) before sending a new Indication. Otherwise, a gEvtError_c event with gGattIndicationAlreadyInProgress_c error code is triggered. The Notifications can be sent consecutively.