Eons ago, the nipple seat diameter was the ERD, but that concept is pretty much gone these days. Most spoke calculators will give you a spoke length value that will have the spoke ending at the same diameter as the ERD value entered.

So it's important that one knows what to expect, and if using a calculator that asks for ERD, either add the allowance for the nipple headto the actual diameter (doubled) to get the ERD, or use the actual diameter, and add the nipple head allowance to the result, which is what I do.

It doesn't matter what you call things, or how you prefer to measure, as long as the builder, rim maker, and spoke calculator are all using the same assumptions, or compensate accordingly.

The problem is that these days, people are doing things differently, but calling it the same, so spoke calculating has become a GIGO process. This is why I and many experienced builders, measure everything ourselves rather than using an unreliable spec. and use the same calculator every time.