Start with the fare classes. You're partly right. Here is an official table with all of the fare classes and what buckets they fit in. And it depends slightly on the route. But, roughly:

• J, C, D: Business (flexible); within Canada/US (C, D Business (lowest) elsewhere)
• Z, P: Business (lowest)
• R: eUpgrade space to J; also SE J IKK space
• I: J reward space
• O: PY (flexible)
• E, N: PY (lowest); N is also eUpgrade space to PY
• Y, B: Latitude
• M, U, H, Q, V, W, G: Flex (M, U, H referred to on FT sometimes as "high Flex" as they require fewer upgrade)
• The rest: Tango
• X: Y reward space

I'm not sure what bucket PY rewards come from. I'd guess N, but someone here will probably correct me on that.

Note that EF doesn't show I space when you search for the flight, you have to do a specific search for rewards seats using that EF function.

Is this the same across other airlines? Not at all. Some are fairly consistent (Y is generally the highest economy fare, for instance), but there are plenty of differences, e.g. A is used by UA for domestic F (and maybe international F too, not sure) and C is the highest business class fare on AV.

As for your flight, all those numbers tell you is what fares AC is still willing to sell, not how many seats are actually available. Remember that AC is willing to oversell on the hope that people will no-show or change to other flights, or, if not, hoping that VDB or IDB comes out to less than they earned by selling a last-minute fare. So, a flight can be Y2 but be oversold.

I'd guess, based on those loads, that it's pretty close to sold out, but not necessarily oversold, since they're still selling a Flex fare (albeit the most expensive kind). But there's no way to be certain from that data. Is it worth going to volunteer for VDB? Could be, but no guarantees. As an E75K though, it's highly unlikely AC would bump you, so volunteering is the only way you'd get paid.