I've been through this recently for fixed wing training.
Previously, I had completed 5 Type Ratings with a AFSP/TSA Approval and by entering the country (USA) on a B1 Visa.
Then, about 18 months ago, it changed to requiring a a TSA Security Threat Assessment. A event-non-specific approval valid for a number of years (Can't remember!).
Recently, I needed a new Type Rating and the Provider advised that I needed an M-1 Visa. I queried this, based on my previous experiences. It turns out that with the move towards Part 142 (instead of Part 61) for Training, an "Initial Type Rating" is an Educational event, not a Business event, therefore, requiring a "Student" (M-1) Visa, not a Business Visa.
Now, a recurrent training event, or additional training of any kind, Differences, RVSM, PBN, whatever, comes under a B1 Visa, but an Initial requires an M-1 if it is conducted under a Part 142 approved training course.
So, I now have B1/B2, C1/D and an M-1 visa, the latter issued only for a specific organisation, and it was a pain to get.
It is what it is.