Formula indicates the chemical composition of the assembly. For example,

A2b4  
means that the assembly is made of two different monomeric units, denoted as A and b. From total 6 monomeric units in the assembly, 2 are of type A and 4 are of type b. Capital letters denote macromolecular monomeric units (protein, DNA and RNA chains), small letters stand for ligands.

Note that unit names in Formula, unlike those in the Composition column, are not derived from chain IDs or ligand names in the corresponding PDB entry or uploaded file. In the above example, type A may be represented by 2 chains with different IDs in the PDB file. Monomeric units are grouped into chemical types on the basis of their sequence and structure similarity. Note that Formula includes all monomeric units in the assembly, including macromolecular ones (protein and DNA/RNA chains) and ligands.