Next, we have a protein with a ligand that is partially occupied (i.e. present in 50% of the protein copies) in a space that does not clash with the apo state of the protein. Compositional heterogeneity is almost always observed in high throughput ligand soaking experiments, which now comprise a huge percentage of the PDB depositions , and is increasing in frequency in deposited cryoEM structures. However, representing this data has been a topic of great debate . In the legacy PDB format, we could encode the compositional heterogeneity by indicating that the small molecule has an occupancy of 0.5 (50%) and provide it with an altloc, often conflated with conformational heterogeneity(Figure ). In the proposed format, we proposed separating compositional heterogeneity from conformational heterogeneity. We propose a compositional state data category that would also be within the atom_site loop and have a dictionary of categories based on the combination of each unique, non-water HETATM (i.e. ligand 1 bound, ligand 1&2 bound, ligand 2 bound, unbound, or blank-indicating unknown, and therefore interacting with all compositional states). For every atom, the user can choose if this is bound, unbound, or unknown for each HETATM. For the ligands themselves, they will always be in the bound state, but for the other atoms around they can be in the bound, unbound, or unknown state. The unknown state will consist of most of the atoms in the protein as it is typically unknown if the atom is in the unbound or bound state, especially in residues far from the binding site. This also allows for a ligand 1 bound to be indicated in the compositional heterogeneity category and pose 1 or 2 in the conformational heterogeneity column.