RedDxb : Have never achieved Plat (and on current projections, am unlikely to do so), but it sounds like a sensible-enough policy to me that you should get F category cars, even if only (?) at DXB where the extensive fleet enables this.

It can however sometimes come down to demand and availability, and I have on the odd occasion been given an F car when travelling J, and indeed vice-versa. Never really bothers me either way, simply because I've always regarded the CD service as simply a convenient, hassle-free means of getting to/from airport and home/hotel.