Open nomenclature involves the use of abbreviated taxonomic expressions in biological classification. The most common expressions used are aff., cf., ?, and sp.. Although there can be variation in where researchers place expressions such as aff. and cf. in the Latin name of a species, the way they are interpreted is the most significant unsettled issue. The International Code of Zoological Nomenclature (ICZN) does not make reference to open nomenclature, leaving its use and meaning open for interpretation by taxonomists.
The expression aff. is generally used to express affinity of a potentially new but undescribed species with a known species. To indicate a potentially new species without showing affinity, sp. is used. This suggests that identification has either not been attempted or the specimen cannot be closely related to established species or subspecies. The expressions cf. and ? signify varying degrees of uncertainty.