Collections are only for trues movie sequels.

How is a 'sequel' defined? Take, for example, Director Ang Lee's 'Father Knows Best' trilogy, or Krzysztof Kieślowski's 'Decalogue'. Even though the characters in each film may differ, they all revolve around a series theme and can be considered a collection.

This is explained in the rules here.

I understand the rules, and I raised the questions and the previous examples precisely because there are ambiguities. The two series I mentioned are not sequels in the traditional sense, but they can still be considered collections.

The Decalogue (1989) is a valid collection due to this part of the rule : "Series of stand-alone films and mixed series marketed as proper sequels are also allowed as collections. These sequels are usually numbered and share a similar theme and/or an universe.".

I don't know the "Father Knows Best" movies. So, I can't answer for these ones.

The collections I added share a similar theme but are not numbered, and are still considered collections on other platforms. I hope they can also be considered collections here. Thanks anyway, I appreciate your explanation.

Collections on TMDB are a unique, self-defined concept. So Collections and other terms platforms such as IMDb/Criterion/MUBI use to group films may be completely different from TMDB Collections.