Yep everybody stay calm and just read.
The most important to understand F18 is the restriction of the second paragraph of the rules guiding principles.
Such second paragraph does not exist in A class or C class which are development class. F18 is a restriction class.
I hope you get it now.

FORMULA 18 CLASS RULES GUIDING PRINCIPLES:

The box measurement rule allows manufacturers to develop catamarans that are competitively priced yet allowing freedom to builders to develop higher levels of performance. Being open to any manufacturer allows many builders and sail makers to compete and so keep costs to a minimum.

The Class remains aware to keeping development under control, maintaining a good balance between cost and performance. Corrected crew weights allows fairer racing with more ladies involved as helms and crews.

If that is not crystal clear enough (OB for ever ), just read that:

PART II – REQUIREMENTS AND LIMIT A TIONS

The intention of these Class rules is to ensure that the boats are as alike as possible in all aspects affecting performance as F18 is a Measurement Controlled Class. The crew and the boat shall comply with the rules in Part II when racing. In case of conflict Section C shall prevail.
The rules in Part II are closed class rules where anything not specifically permitted by the Class rules is prohibited.

Developpment under hard control indeed