The best analogy really is a series of releases / recordings that real-world labels do.
For example, Topic Records, the UK’s oldest independent label (primarily folk music) made a series of volumes, these were called ‘The Voice of the People’ and are numbered volumes.
Here is the series on MusicBrainz: The Voice of the People - MusicBrainz
In this case, the majority of the series was made up of recordings of a variety of people, each volume themed. However, they made one volume which was credited to just one artiste.
Anything that is made up of numerous separate releases, with some kind of logical theme or order (usually numerically ordered, but not necessarily), is a good example of a series.
The key thing is that there is something ‘official’ that distinguishes them and forms a connexion, maybe the label (or artists / collective of artists, in the case of self-released items) brands them or they use it in marketing to highlight something that links them. It’s not the same as an actual label because usually a [sub]label has some degree of separate operation, or name indicated, or similar. It’s more complicated if the label uses “Series” in its name (e.g. the ECM New Series) but usually the context reveals the true intent.