Capturing ensemble information in the legacy PDB format becomes an even more complex problem when considering compositional heterogeneity, which can coexist with conformational heterogeneity. Compositional heterogeneity is often observed with ligands bound at sub-stoichiometric occupancy in X-ray structures and with different components in large macromolecular complexes in cryoEM . Compositional heterogeneity is captured using the same "altloc" column as conformational heterogeneity. This ambiguous representation inhibits disentangling compositional and conformational heterogeneity, especially for large data mining efforts.