Here, we proposed amendments to the PDBx/mmCIF model format to improve the encoding of the conformational and compositional ensembles in experimental structural biology data. Using the extensible and flexible dictionary-based data structure of the mmCIF/PDBx format, we propose separated entities to capture conformational and compositional heterogeneity that can be layered to show hierarchical relationships (Figure ). These modifications will improve our ability to explain structural ensembles and provide critical training data for new protein ensemble-function predictions.