if I may:



I think what Sam means is, that you can make modifications to a Tiger which are perfectly legal under ISAF/NAF-18 rules. However those changes might not be in compliance with the the IHCA Tiger (one-design) rules.



So if you were then to show up at an event which was being governed by the IHCA Tiger class rules, you might have to change those modifications back, to avoid an equipment protest.



A good example might be that you decided to put a non-Hobie (but properly sized) jib on your Tiger. That would be fine in an F-18 event. But other Tiger sailors might object, in an event being run under IHCA Tiger (one design) rules.