Quick DDD terminology question: Say Context B depends on A (e.g. because B uses one of A’s services). Is A always “upstream”, or does this change depending on whether one applies “Conformist” or “Customer/Supplier”?

Btw the question phrasing seems odd, as you use "depends on" and "uses service" as synonyms. Runtime dependency is independent of model flow imho. B can call A, but if A conforms to B's model, B is upstream, A is downstream.

Don’t introduce the term “model flow” again, I carefully tried to avoid it ;) Of course run time and development time dependencies don’t have to map 1:1. Let’s just assume they do for this discussion.

But "dependency" is confusing: To me, in a context map, "B depends on A" would mean A drives change in B, e.g. A is upstream, B is downstream. But you seem to mean something else with "depends on".