JAV

JAV mean Japanese Adult Video.

From a bit of observation the letters seem to come from a mixture between Studio, Label, Director, and possibly title and actress too, but I don't read Japanese so I can't confirm the last three although I'm certain on the first two and leaning towards saying the Director sometimes plays a role too.

Series of course have the same letters which is because the first 2 things I mentioned usually doesn't change, the Director might (although I think they would just release it as a new series but I'm not sure), and the title changes a little sometimes only with a number and sometimes rephrasing the original title to match the new episode.

The number?
Could be the studios release number.

It's the magic key to find torrents