I created the following Filter on Software Engineering:
I expected the tags to be joined with OR - a question with at least one of those tags without an accepted answer would show up. However, after experimenting, I've found that it appears that the tags are joined with an AND, meaning that a question must have no accepted answer and have all of the listed tags to show up.
As far as I can tell, there's no way to save the OR in this filtered question view, even though it works for searching. Or perhaps it's just difficult, since "or" shows up in tags that are provided by autocomplete, but I haven't found a workaround yet.
It seems like an OR is a more reasonable default for tags listed here.
So I poked around more and it looks like the easiest solution is to edit the search and save that as a filter. There isn't autocompletion in the search bar at the top, so you can execute a query, add your ORs, and then save the search as a filter. But this is not intuitive and adds a lot of extra steps. The interface is not very clear as to what is happening with the filters or how to actually make filters that do what you expect or want.