ProtoSMILES sets no limits on the atoms that can be marked as selectable (i.e., "aromatic"), but Balsa does. As described previously, pruning the DS requires a method to determine whether or nor an atom is subvalent (see: Valence and Subvalence). Making this determination requires at least one default valence. The only reasonable way to allow selection of all atoms regardless of associated element would be to provide default valences for them. As a result, only those atoms associated with a subset of elements may be selected ("B", "C", "N", "O", "P", "S"). This list is a subset of the elements whose default valences are defined. Halogens were excluded because they rarely appear in rings. This restriction only marginally limits the capabilities of Balsa because atoms other than those supported are rarely involved in DIME.