The PDBx/mmCIF is an extensible data representation built on a flexible dictionary-based system While this data format allows for a more robust representation of many structural models, conformational and compositional heterogeneity is encoded in the same way as the legacy PDB format (altlocs and B-factors). However, conformational and compositional heterogeneity represent two very different types of heterogeneity. Conformational heterogeneity refers to different conformations of the same species (i.e. different atomic coordinates) within the same dataset, where compositional heterogeneity refers to different species (i.e. small molecules or monomers of a macromolecular complex) within the same dataset.