Huh. I figured this would have a quicker answer.

Corellian Engineering Corporation has been around for centuries, possibly millennia, building starships all the while. Their products include B-, C-, D-, F-, G-, H-, K-, L-, S-, T-, V-, W-, X-, and Y-series ships. I can only assume that "YT" is what came after "YS" and before "YU" in their design process.