The Gnus registry is a package that tracks messages by their Message-ID across all backends. This allows Gnus users to do several cool things, be the envy of the locals, get free haircuts, and be experts on world issues. Well, maybe not all of those, but the features are pretty cool.
Although they will be explained in detail shortly, here's a quick list of said features in case your attention span is... never mind.
This keeps discussions in the same group. You can use the subject and the sender in addition to the Message-ID. Several strategies are available.
gnus-summary-refer-parent-article can take
advantage of the registry to jump to the referred article, regardless
of the group the message is in.
The registry can store custom flags and keywords for a message. For instance, you can mark a message “To-Do” this way and the flag will persist whether the message is in the nnimap, nnml, nnmaildir, etc. backends.
Through a simple ELisp API, the registry can remember any data for a message. A built-in inverse map, when activated, allows quick lookups of all messages matching a particular set of criteria.