The official guidelines for ICD-10-CM define the conventions used in the code set. In section 1.A.6, we learn the following about some common abbreviations in the Tabular List:
NEC “Not elsewhere classifiable” This abbreviation in the Tabular List represents “other specified”. When a specific code is not available for a condition the Tabular List includes an NEC entry under a code to identify the code as the “other specified” code.
NOS “Not otherwise specified” This abbreviation is the equivalent of unspecified.
Section 1.A.9 tells us a little more: