What does "to be counted outside the 'three plus one' cap" mean?

Question

Japan has made these moves far easier for players from the ASEAN region by a ‘visa exemption’ that allows nationals from most of the zone to be counted outside the ‘three plus one’ cap that otherwise applies, and that’s seen several others enjoy brief, if unsuccessful, stints across the top three flights of the country. (Source)

I don't really get the meaning of the sentence above, especially the part "to be counted outside the ‘three plus one’ cap that otherwise applies, and that’s seen several others enjoy brief, if unsuccessful, stints across the top three flights of the country"

I'd really appreciate it if you help me understand it. And it would be helpful if you paraphrase it in the simplest way.

Answer 1

This link explains what the three-plus-one rule is: basically a Japanese football team can have a maximum of three players from outside the ASEAN region and a further player from within the ASEAN region.

A rule that places a limit on something can be called a cap.

If somebody is counted outside the three-plus-one cap, it means that they are not counted within the three-plus-one, and so the cap does not apply to them.

A stint is a period of time doing a particular job. The top three flights of the country may be a (somewhat unconventional) reference to tiers J1 to J3 in the Japanese Association Football League.