We propose extending the PDBx/mmCIF model to include new conformational and compositional data categories linked to atom-level data (Figure ). Additionally, both data categories would be hierarchical. For each atom, the first conformational state would represent the base or first layer of heterogeneity, with subsequent states explaining heterogeneity 'within' the previous state. For example, in the conformational data category, conformational state level 1 could contain information on a loop state (in or out), whereas conformational state level 2 would contain a backbone peptide flip in the out loop state, and conformational state 3 could contain information about a side chain alternative conformation that occurs in a residue in the out loop state and with the peptide flip. This layering would allow for the knowledge of hierarchical conformational heterogeneity. Isotropic and anisotropic atomic displacement parameters (B-factors) would still exist in the atom-level data across the hierarchy. It will be straightforward to continue refining TLS-derived atomic displacement parameters for non-multiconformer regions; however, the exact implementation of how TLS-derived groupings will co-exist with hierarchical representations of conformational heterogeneity will require careful attention in refinement software. Separating compositional and conformational heterogeneity into individual data categories allows us to understand how they are linked, such as multiple conformations populated in the liganded state. Importantly, the inherent flexibility of the mmCIF format paves the way for a standardized and adaptable format to capture conformational and compositional heterogeneity depending on the experimental data.