The discussion was tentative and I don't think we have something solid enough to serve as guidelines.
*Generally*, noticeable bodies of knowledge are marked by Familiarities and Expertises in the DCH entry (our crowning moment remaining Ms. Marvel's Expertise with her soup recipe, though it was mostly there for colour and slight humorous value), and between these and reviewing the professions that a character has worked in I think there's sufficient info to assign Expertises.
The one solid exception remains Connection (Street) and in some cases Connection (Underworld) mapping to an Expertise, since it was it what inspired the Connections -> Expertises line of thought.
But for the rest, the use of Expertise in Green Ronin writeup seems to be "assign an Expertise based on job, declare victory, and be liberal in allowing to roll under it when it comes to vaguely related subjects". Which I think is sound, since Expertises cost a fair bit and having a/ a broad interpretation and b/ few Expertises per character aligns well with what Player Characters built using points can afford. Yes, we don't quite care about points for NPCs, but let's not go completely off the reservation in that regard.
I think my point can be summed up as "look, if Batman only got a whole *two* Expertises...".

