Appendix 'A' says that an NoR or SI for a series may provide for throwouts. I presume this means that if say 10 races are started and at least one boat finishes, the race is "scored" and therefore counts, and that a "score" can include a DNC. So if we do 10 races, and two throw outs, then a boat that started only 8 races can throw out two DNCs. Is that correct?
Next question, what if each race in a "series" has its own NoR and SI, but there is none for the series?
RRS 90.3 explains it well. I believe you're on the right track
Appendix A doesn't say that at all.
Appendix A RRS A2.1 says
Each boat’s series score shall, subject to rule 90.3(b), be the total of her race scores excluding her worst score. However, the notice of race or sailing instructions may make a different arrangement ...
Yes, RRS 35
Yes
RRS A1 says
The number of races scheduled and the number required to be completed to constitute a series shall be stated in the notice of race or sailing instructions.
https://static1.squarespace.com/static/62b8b53ddecd3441cbee091d/t/646e385d75f17f2ddeb1ef12/1684944989800/IITS-NOS.pdf
If you don't have some kind of document (bowing to Matthew Curtiss) which will govern your series, you don't have a series. In your Notice of ... or SI, you must spell out what constitutes your series. If you want more than 1 throw out or multiple ones at different levels, it has to be included. You may want to consider a different scoring system other than low score, which is the default. Especially when you are including multiple events in the series. I know of at least one club that has a long series and they use high-score.
So the ubiquitous, poorly conceived NOR or SI line, more or less like "One completed race constitutes a series" means nobody scores at all if only one race is completed.
Here in the Chesapeake Bay, the CBYRA (Ches Bay Yacht Racing Assoc) uses “High Point”, which allows for season long scoring across multiple events by MANY diff YC’s/OA’s.
Here is a description of how it works.
The above is the junior-Sailing version. Each class has their own HP series. For other big boats, the “bonus points” are allotted to distance races greater than 20 nm.
Here are the High Point rules for the J105 Class.
It's hardly a new 'catch'.
Exclusion of a boats' worst score has been in the scoring system since the RRS first introduced a scoring Appendix in the RRS in 1969.