Title says all. But . ..

1/- Under what conditions would I use it?

2/- How is this more advantageous than Manual Mode? When in MM I have GAIN and aperture too? What's the big deal here?

3/- Why call it Av when it is also called Aperture-Priority - surely AP would be more precise? - I suppose it means Aperture Value?

. .still learning .. . 2½ years later!

Grazie

Good question.

"Aperture priority" (Av) exposure program mode is basically a hold-over from Canon's still photography legacy. While Av exposure mode is a mainstay for still photographers it's not very desirable for videographers, as it leaves shutter speed as the exposure variable...something that can produce unwanted effects in video footage. "Shutter priority" (Tv) mode is the mainstay program mode for videographers.

The labels for these modes are Exhibit A for why you should never let engineers establish the public designations for anything. "Av" = "aperture value" and "Tv" = "time value" (the shutter's exposure time, presumably).

Since I shoot indoors under available light, I shoot Av "wide open" all the time.