The order can be adapted to fit different use cases depending on the curation rules and data structures of the individual resources. For example, the substitution of "P" with target "T" can be used to indicate a cell or a microorganism, an SAR series can be represented as a multiplexed set of one-A to-many R-C. It can also be extended to "D-A-R-C-L-P" where L refers to the explicit location references for C in the document (e.g. "compound 10b" in a paper or "example 503" in a patent). However, it is limited as a formalism for bioactivity since there are many exceptions and mechanistic nuances that do not fit this simplification. A well-known example would be heparin (GtoPdb ligand 4214). In this case "C" can be a commercial partially purified extract of 12-15,00 Mw which consequently does not have a defined chemical structure. However, as a curatorial expedient, the defined 1040 Mw form (as PubChem CID 22833565) has been annotated, even though the sodium salt is the active form in vivo. Note also that while formally "P" is SERPINC1 (ATIII) the mechanism is an indirect one involving the activation of binding to F2 (activated thrombin) for inhibition. Another example of a shortcoming is mechanism-based covalent inhibition where the time dependence of IC50 is not captured.