Are you sure it's not 3AA in the first line? It would make more sense and would be the flap. Size 3 and forward. AR would be a new type of flaps, and you would need to call antares because I don't know what those are...
18 is the seat size, XL I'm assuming is some sort of extra large seat that was requested by the customer.
AR+10 means the back of the panels is lifted slightly, a pretty standard fit.
Everything that Maytay02 said is correct about the other numbers.
Thanks so much for your reply! Yes, I had also never seen "3AR". According to the owner, it means that an extra inch is added to the width of the flap, so it is a wider flap? Anyone know if this would be considered a "long" flap? I am 6'2", but not too long in the leg, so a normal long flap should be what i need. Thanks!