A1 and A2 are analog input channels on compatible telemetry receivers.
A1 is most often used to report receiver voltage, and A2 pack voltage. A1 and A2 can report any value that can be represented as an analog voltage between 0 and 3.3 volts. (D8 receivers- A1 and A2 available. D4 receiver only A2 available. X8R receiver A1 always reports Receiver voltage, A2 not available. X6R receiver A1 available)

A3 and A4 are values reported by the Smart Port to UART Converter. A3 and A4 can report any value that can be represented as an analog voltage between 0 and 3.3 volts.