F17, in my opinion, is exactly what you suggested; a confusing sales pitch. I suppose that maybe it's an attempt to start a new formula class - but unlike F18, F17 is an initiative started by a single manufacturer (and unlikely to succeed in this regard). Personally, I have not seen any "F17" measurements or rules as it pertains to the formula. Also of note is that the I17 platform saw a large round of general modifications when the boat went to being referred to as F17 (clever disguise?). A lot of these are desireable performance or manufacturing reliability modifications but they caused a stir within the already existing I17 class.

The Nacra F17 is a speedy single hander - but there's not really a 'formula' 17 class in existence nor have I heard of much push to create one.