A multimeter measures current in amperes and potential difference in volts. Wattmeters are used to measure watts and the reading is a combination of current being drawn and the voltage applied. watt = volts x amps

You can test a battery by using multimeter. Set the multimeter to the DC voltmeter setting and then place the leads of the multimeter across the leads of the battery. the multimeter will have a readout of the voltage.

An automotive multimeter, like any multimeter, can be used to test the voltage or current in some kind of circuit. As it is an automotive multimeter it is aimed more towards the used in a car.

The analog multimeter has a wider range.

Yes it is possible to test a diode with a multimeter.