Bzzy18 covered a lot of good ground, but for more context…
2A flap = regular length, forward flap. That said, it’s highly unusual to see less than a “A” flap on a 17.5" or 18" model. So even though 2A technically means “regular length and forward,” it’s a pretty standard, vanilla flap for that seat size.
B10 = 10 mm aka 1 cm of foam added to the B zone
D10 = 10 mm aka 1 cm of foam added to the D zone
When you see a Voltaire with equal added padding in the B zone and the D zone, it’s typically been customized to fit a horse with a flatter topline. The Voltaire trees are pretty curvy, so the only way to get the panels to lie flat(ter) is to add foam to the front and back. The PRO panel is a nice complement to this since the PRO panel is beveled to lie flatter against the well-sprung ribs that many Warmblood sporthorses have these days.
It’s hard to explain in words how much more forward 2AA is than 2A. But at 5’9" and a 34" inseam, the flap length is the thing that caught my eye. A Voltaire 2 flap is usually around 13.25" long, although that can vary slightly by saddle model. But t your height and inseam length, it’s not unusual to need a 14" flap. But it’s hard to say since everyone’s leg is different, people ride at different stirrup lengths, etc. If in doubt, measure your current saddle flap length and use that to triangulate what you need.