@gerardPolloRebozado said:

Im trying to use the discover API for movies and tv and there ar e many fields that im not sure what they do.
For example whats the difference between watch_region and region?

watch_region
Only effective when used in conjunction with with_watch_monetization_types or with_watch_providers.
https://developer.themoviedb.org/reference/watch-providers-available-regions

region
Refines the response to just one region, i.e. one country,
Basically filters for Certification.

also it doesnt specify in what format should I write teh region.

Value format.
ISO 693-1 - 2 lowercase letters for languages.
https://developer.themoviedb.org/reference/configuration-languages
ISO 3166-1 - 2 uppercase letters for countries or regions.
https://developer.themoviedb.org/reference/configuration-countries
YYYY-MM-DD - For dates. (Example from today 2023-07-31)

For example spain should be SP, ES, Spain, España?

es-ES - spanish-Spain
es - Spanish
ES - Spain

or what is the difference between primary_release_data and release_date?

primary_release_date
This is the worldwide release date.
It defines the year/date that appears in parentheses next to the title on a Movie or Series page.

release_date
All the premiere or re-release dates of a Movie in any country in the world, in any format, type or location.
Festival, Movie Theater, TV, Physical (DVD, Blu-ray, etc.), Digital for download, rental or sale, etc.

If I'm not mistaken...
When the region is specified, it returns the premiere date in that specified region.
When the region is not specified, it returns the worldwide premiere date, i.e. primary_release_date