V<single letter>: The voltage associated with a particular thing, i.e. Ve is the emitter voltage of a BJT, Vs is the source voltage, etc.
V<single letter><different letter>: Usually more for theory, the voltage going from something to something, i.e. Vbe is the voltage from base to emitter on a BJT.
V<two same letters>: The supply voltage typically associated with a terminal on a transistor. I.e., VEE is the emitter (almost always negative) supply in a BJT circuit, VDD is the drain (almost always positive) supply in a MOSFET circuit (even though a CMOS circuit will have sources connected to either VDD or VSS, because the transistors are a mix of NMOS and PMOS).
V<two same letters><some other letter>: The supply voltage typically associated with a terminal on a transistor, only special somehow. In your case, VSSA means "lower voltage analog supply rail", and it's mated with VDDA, which is the higher voltage analog supply rail. So VSSD would be the digital VSS, VDDD would be the digital VDD (and I've never seen VDDD, but strange things are out there). It's certainly conceivable that you'll see VDD1, VDD2, etc., on a multi-supply board or chip.
Ais likely standing fro Analog. \$\endgroup\$