Not elsewhere classified (NEC) is a term used in a variety of classification systems to denote an entity that has been fully characterized but whose features are contradictory and/or do not fit into established classifications.
Not elsewhere classified (NEC) should not, however, be confused with not otherwise specified (NOS) which denotes an incompletely characterized entity.
Louis D, Wesseling P, Paulus W et al. CIMPACT-NOW Update 1: Not Otherwise Specified (NOS) and Not Elsewhere Classified (NEC). Acta Neuropathol. 2018;135(3):481-4. doi:10.1007/s00401-018-1808-0 - Pubmed