Yep, it is confusing.
A boat built in 1994 and called a Hobie 18 Formula could be what we know as an SX or maybe even a white painted Hobie 18.
When the SX first came out, the sails had the name "Formula" screened on them within the flying "H" logo. These were produced by Neil Pryde for Hobie Cat Europe as the new model Formula 18 in Europe. At that time Hobie Cat USA had just presented a line of white painted boats and had called them "Formula 1" models, so to limit even more confusion, we called the new vertical cut, mylar and taller rigged boat a Hobie 18 SX.
If the sails are vertical cut and mylar... it is an SX.
Director of Parts and Accessory Sales
Hobie Cat USA