primary_release_date
It is the year of the worldwide release of the film. The one that appears in parentheses next to the title.

release_date
It is the year that it was premiered in some country. It is not necessarily the worldwide launch.
Or the releases on TV or Bluray.

WRONG If you, in your API Request, specify a region, you will receive the release date in that region. WRONG

For example this film:
https://v5.themoviedb.org/movie/11832-babettes-g-stebud
It was released worldwide on 1987-08-11 (Norway)
But in the USA it was released on 1988-02-04

The same date appears whether or not you specify Region.
https://api.themoviedb.org/3/movie/11832?api_key=###API_KEY###&language=en-US&region=us
https://api.themoviedb.org/3/movie/11832?api_key=###API_KEY###&language=en-US
The release data in both appear as 1987-08-11.

The "get details" methods do not support the region filter. You'll need to call the /release_dates method and parse out the region and type you're interested in yourself.

https://api.themoviedb.org/3/movie/11832?api_key=###&language=en-US&append_to_response=release_dates

I'm already doing this way.
ticao2 pt-BR answer:

If you, in your API Request, specify a region, you will receive the release date in that region.

A thousand apologies. I made a mess.
I hope your question has been answered.