Question: What does major mean in a Dialog thesaurus?

Answer: 
This option, when selected, will search the selected term within the thesaurus.


Major is assigned to a term (by the Embase & Medline indexers) to denote that this describes what the major thrust of an article is.

Using “Drug Therapy” as the example:

MJMESH.exact(“Drug Therapy”)  or for Embase MJEMB.exact(“Drug Therapy”)

An example record retrieved is:

Title: Efficacy and safety of acupuncture combined with western medicine for anxiety: A systematic review protocol


As you can see, for every term that has "major" assigned to it, (major) is listed at the end of that term.

Unfortunately, when you instruct the Embase/Medline thesaurui to restrict to “Major”, it will only search for terms limited to that particular designation.