I'm not sure, but I'll take the risk:

Example:
primary_release_date > 2015/09/25 (in England)
primary_release_year > 2015
release_date         > 2016/03/14 (in France)
Year                 > 2016
release_date         > 2017/01/12 (in Brazil)
Year                 > 2017

Date and Year: The data is the same.
The difference is the way the data is presented.
The full date (2015/09/25) or only the year (2015).

The difference is between primary and non-primary

Primary is the oldest date of the movie in the DB.
It is the year that appears next to the title, in parentheses.

Non-primary are the dates, any date, release in all countries or languages.
That is: 2015 in England, 2016 in France and 2017 in Brazil.

@ticao2 is correct, primary comes down to a single data that we hold on the item. What and how we calculate the primary release date is explained here. Otherwise a release date is just one of the many that is added to a movie/TV show.