The symbols stand for: A (amplitude modulation), 2 (one channel containing digital information, using a subcarrier), A (aural telegraphy).
Amateur radio operators may also use A2A to transmit Morse code, however it would be more typical to send it as CW, rather than MCW, using single-sideband modulation with a suppressed carrier, which reduces the bandwidth and power requirements.
Unlike A1A CW transmissions (double sideband, with no subcarrier), A2A MCW will produce an audible audio tone from an AM radio receiver that is not equipped with a beat oscillator.
The FCC designates this mode as F2A: F (frequency modulation), 2 (one channel containing digital information, using a subcarrier), A (aural telegraphy).
F2A MCW Morse can be heard on a normal FM radio receiver, and it is commonly used by both commercial and amateur repeater stations for identification.