Here is a brief description of what these terms mean.
Sensitivity means that a broad based search of the literature is desired.
The researcher wishes to pick up as complete a retrieval as possible. Searchers might wish to do this if they are getting too small a result from their search. Searching using both textwords and MeSH headings. Not using subheadings to narrow a search. Specificity means that you want to narrow the search results to zero in on a specific topic.