IM & NIM Terms
After finding all the MeSH terms correctly describing the content of an article, the indexer must also weigh the importance of these terms. For example, an article describing kinetics of lactate dehydrogenase, will be indexed with both LACTATE DEHYDROGENASE and KINETICS. However, the main point of this article is LACTATE DEHYDROGENASE, while KINETICS is simply a minor point, which further specifies what is happening with LACTATE DEHYDROGENASE. This distinction between the main and minor points of the article is designated as IM for the main point, and NIM for the minor point.
The origin of the designation IM and NIM comes from the time when terms describing major points of the article were printed in Index Medicus. IM meant printed in Index Medicus and NIM meant not printed in Index Medicus. IM terms are the main point of the article, while NIM terms are secondary terms supporting the major points of the article. IM concepts are designated by an asterisk (*).