A "research" search is when the user is looking for documents on a topic. The user may not know if any documents on the topic even exist. The search is typically for keywords in the document.
An example is: information on "arms-dealing".
A user will expect a list of documents displayed by relevancy.
An enterprise search tool excels at this type of search. The user is looking for terms in the document. The enterprise search tool can use its algorithm to identify which documents have the most treatment of the search terms.
A typical DMS will fall short on a "research" search. A typical DMS does not rank searches based on relevancy. If a search yielded dozens or more results, the user would have no reference as to where to start a review of search results. A typical DMS also has an inferior text search engine.
It is the frustration when running a "research" search that users cry out for an enterprise search tool.