A WordPress plugin that provides an admin UI for assigning a collection of posts to another post, and an API for fetching collections.
A post collection is a manually curated list of posts that gets attached to a single post. The collection can then be displayed by the theme however it sees fit. A post collection is similar in construct to ACF's post relationships, except post collections uses a taxonomy for the relationship so the collection can be queried in both directions (fetch the posts in this post's collection; fetch the post that owns this post's collection).
Support for post collections should be added to a post type via the post_collection_types filter.