The BT136 is TRIAC with 4 A maximum terminal current. The gate threshold voltage of the BT136 is also very less so can be driven by digital circuits. Since TRIACs are bi-directional switching devices they are commonly used for switching AC applications. To switch off control of an AC load that consumes less than 6 A with a digital device like a microcontroller or microprocessor then BT136 is the best choice.