In order to apply for International Status as a Race Official (Judge, Umpire or Race Officer) you need to have enough experience. The ROC considers your application and looks at the events you have been participating as an official. The guideline if those events are "good" enough is described in a publication from ISAF called: Principal Event Guidelines 2007
There is no definition for "Principal Events" because that would tie the hands of the committee to much. From the text is is clear that the exact criteria are open to interpretation and that is in my opinion a better way than to insist on participation in events specifically named. The experience counts, not the title of the regatta. If you have doubt if your event counts or should count, you can always ask the chief Umpire or the Chairman to give you an assessment of the level you've been exposed to. I've been to National Championships with 50+ boats and 7 regatta's with NO protest at all and also to Club-championships with more than enough work to satisfy any international jury. If you are thinking of becoming an IJ, IU or IRO, start keeping a record of all your events, and include as many details as you can.
I use a short database for that, so that I can filter on any criteria. But don't leave anything out, the more experience you can show, the easier it is for the committee to decide in your favor.
New Principle Event Guidelines 2008 is published on the ISAF site.
As far as I can see there are no changes to the one in 2007