It's probably not the easiest thing to define, but one of the requirements for my local database is to define if a series is linear or not. Currently I am defining NON_LINEAR, LINEAR, PARTIALLY_LINEAR, and UNRELATED.
NON_LINEAR and LINEAR probably speak for themselves. PARTIALLY would require a link from one episodeId to another, to define things like part 1's and part 2's. UNRELATED would be things like sketch comedy, late nights, etc where episodes have no relation to each other. Another good example would be the crossover shows like Homicide and Law and Order, which had connected shows.
I'd prefer not to keep that information within a local user, and even worse would be to maintain an extended database based off of the TVDB, which is due to my lack of knowledge and the fact that it probably would violate LA. I imagine if the field was there, it could be populated over time, but what do I know? For now, it might be nice just to have a placeholder.