Hat tip: Great follow-up. I asked Google AI about those visa types and it told me:

    > F, M, and J visas are all nonimmigrant visas for foreign nationals seeking to study or participate in exchange programs in the United States.

Tangent: US must have 5x the number of visa types compared to most other highly developed nations. Whenever there is a US visa discussion on HN, I always learn about a few more types! Plus they have all sorts of weird carve-outs for various nations: Looking at you Australia and Singapore.