@Julian yeah that's a good point - I think tag would work but I always got the impression that they were more of a micro-level part of a specific piece of content vs. macro-level (being the place they're being discussed in). context would be set at the forum level (whatever the forum's owner specified) whereas tags might be decided by the user. So in that sense, context would more about origination of a post.
Example might be something like this -
{ id: "https://yuforium.com/forums/cars", type: "Service", name: "Example Forum About Cars", context: "https://another-instance.org/topic/cars" }Where a POST to that forum's outbox with a Note would result in that note's context defaulting to the one set to the forum. Posting with another context would result in an error. This makes things a little different than a tag which is what would be user specified. In that sense, context is more about where the post was created, vs. what it was created about. In a federated system, where could be an authoritative entity that encompasses multiple instances and is dereferenceable, or where could be defined as a UUID and be completely unauthoritative and ephemeral.
Given that it's more about the origination of the post, I would agree that the term "Community" is better in this case to define what context relates (instead of "Topic") so in the example, context could be switched to https://another-instance.org/community/cars.
It's been a while since I wrote that up, and at the time I was considering using "Community" as the terminology for a context so I might update that soon (especially with "Topic" being a frequently used convention in forums meaning something totally different).
In the Activity Streams docs, the one part about context that got my attention was this - "An example could be all activities relating to a common project or event", meaning that context exists outside of the scope of what a thread would be, and is more indicative of a forum level or federation setting vs. the contents (objects, activities, etc.) of a single thread.
