We, eg the catsailing community through US Sailing Mulithull Portsmouth Committe define a boat class as a specific configuraton of sails, boat weight, crew etc for the convenience of us all. The PN committe follows the lead of the builder or the class association. These groups spec the boat out and on that basis results are collected assuming all of the boats are about the same and a fair rating calculated. The assumption is to have a level playing field and by specifying the critical speed components (sails weight etc) and putting a name to them, eg Nacra 5.8 or P19MX, we will have an easy time calculating relative finish positions fairly. So, you the sailor agree to not change anything that is a speed factor (sails boat weight, etc).. Of course you can replace tired sails with fresh new ones ... and WE ASSUME you are doing so regularly. (You can't complain that your 10 year old sails prevent you from sailing the boat to its number and you want a correction for that)

So, what happens when the configuration evolves because you decide a new square top main is a better sail for you then a new pinhead main? Rather then say... NO... go home we can't race against your modified one of a kind boat fairly. We (all of the racing sailors) agree to use a set of modification factors that correct your newly modfied P19. If a group of sailors agree to a set of modifications, spec them out and give it a name... then the PN committe collects data on that configuration and issues an approved rating. (best example is the Nacra 6.0 NA with NE spin)

One restriction is that in the regional championship regatta for the US Sailing championship you are restricted from using your one off modified design. So, for the Alter Qualifier in St Michals, MD which is your sailing region you would have to use either the P19 sail configuration... (pinhead main small jib) ... OR the MX configuration Squaretop main and large jib). For you club regattas... you can calculate a set of ratings that will be fair to you and to your competitors for whatever configuration you have.

Hope that helps
Mark