Substance indexing SPL files are capable of describing a wide variety of substances ranging from small molecules to botanical abstracts. This capability will equally be present in Reaction SPL files though atomic description of the reactant and product molecules, or at least the part of it relevant for the reaction in the case of bio-macromolecules, will typically be required for a meaningful description of the reaction. One central part of the modular data model is that of a "moiety." A moiety can be any part of a substance. It does not have to be a complete functional group. It does not have to be covalently connected to other moieties. There are two types of moieties: Additive moiety, which contributes to a whole complex substance; Site of interest, which delineates features or sites of interests, such as amino acid connection points. The moiety is a "simple chemical" is used to describe small molecules. The structure of this moiety is represented by MOLFILE and/or SMILES. Small proteins and nucleic acids (up to 999 atoms and 999 bonds) are also represented as simple chemicals. Importantly, InChI is required for unique identification of the structure. InChI's canonical atom numbering is used as one central feature in the SPL description of molecules. For example, the definition of substituents relies on InChI canonical atom numbering (Fig. ) Figure . Definition of substituents via InChI canonical atom numbers.