A "fetch" search is when the user has a document ID (with a Document Management System) or a file name (with a file server system).
The user would expect the single document to be returned. There should be no need for relevancy rankings.
The "fetch" search is the most basic of the four types of searches. It is such a basic part of a DMS and works so well in a DMS that most users do not even think of it as a search. Nonetheless, it is the most common search and the most important. A user expects to be able to get a specific document back instantly for editing or reuse, without having to interpret search results.
A "fetch" is core functionality of a DMS. An enterprise search tool would fall short in this type of search. The DMS is keyed to find specific metadata from the document profile. The enterprise search tool typically uses the metadata to influence the relevancy rankings of a particular document.