Rule of Three
Indexers always try to select the most specific term that describes the concepts in an article. However, if more than three specific concepts treed under a more general concept common to all are discussed in an article, index the general concept (IM) and specific concepts NIM or not at all, depending on the article.
For example "Levels of D-aspartic acid, 1-carboxyglutamic acid, polyglutamic acid, and isoaspartic acid." can be indexed as AMINO ACIDS, ACIDIC.