Erdos Problem #593 asks which finite triple systems occur in every uncountably chromatic triple system. The answer is exactly the class generated from private-vertex expansions of finite bipartite graphs by finite disjoint unions and one-point amalgamations. Problem #1177 on exact spectra is settled alongside it.

Erdős Problem 593 (500): Characterize those finite 3-uniform hypergraphs which appear in every 3-uniform hypergraph of chromatic number $> \aleph_0$.

A natural conjectural characterization, recorded here, is that the obligatory finite 3-uniform hypergraphs are exactly the 2-colorable ones (Property B). The forward direction (IsObligatory → IsTwoColorable) and converse (IsTwoColorable → IsObligatory) are stated as separate variants below; in the graph case ($r = 2$), Erdős–Galvin–Hajnal [EGH75] proved the analogous result (obligatory ⇔ bipartite).