Kris has it.

When you enter a regatta... PN or SCHRS... you declare your rating. If you have a class legal boat.... your rating is in the table and shared by everyone else.

If you have a modified one design boat.... In portsmouth.... you declare your adjustments and calculate the 5 ratings you need using the portsmouth modifications table and present those ratings.

If you have a home built or one off design, The PN committe will guestimate your rating prior to the event and you provide that rating.

In SCHRS... you report your measurement certificate rating for your modified or one off boat which measures the rated changes you have made on your modified cat.

Some regattas... allow no adjustments to boats which effects their ratings....eg Area Qualifiers. Other events use the full PN modification table including weight adjustments.... eg SHBC Statue of Liberty race.

Substituting SCHRS will be simpler (1 rating). No difference here.

For the purpose of this debate... Which is to just change from PN to SCHRS for AREA QUALIFIERS.

There are two current restrictions on entries.

Provisionally rated boats... (eg the Nacra F20).
Modified one design classes. (eg P19 with square top main).

The restrictions on entry exist BECAUSE of the known failings and limitations of the Portsmouth PN System.

The provisional ratings are not trusted because the data is sparse. So the boat is not allowed.

Modifications are not allowed because the modifications are simply educated guesses to the magnitude of the performance gain or loss. These modifications are even more difficult to statistically measure then stock boat performance and may not translate between a hobie 16 and a Nacra 20. The consensus is that a bogus modification on a bogus Portsmouth rating was simply a bridge to far. No one believes the rating was fair. So... these boats are excluded from the Area championships.

The Alter cup committee will have a simpler job under SCHRS racing.

There will be no provisionally rated boat status. The nacra F20 has a valid unchanging world wide rating today.

Modified boats can be excluded by fiat as we currently do.

OR the Alter committee could allow modified or One Off boats to race with a valid ISAF certified rating.

Or they could allow a modified one design boat to race with a USSA Multihull measurer approved rating.... (Scooby's suggestion)

No matter what the outcome... it will be the boat owners responsibility to get the paperwork needed to race his odd ball boat in Area Championships.

As PGP noted... its a Corinthian sport.... for your local handicap regatta if you increase your main sail size... you should measure it according to the diagrams.... calculate your rating... Declare and use it when you race.