Shelf’s filters are there to allow users to easily organize and sort through the content they add to Shelf. The many filters allow for users to narrow their search results especially as the number of files you have in Shelf begins to scale upward in size. For example if I wanted to find a document about chemistry that I have saved in Shelf I would start by running a keyword search for Chemistry. However this search might return too many results of content relating to chemistry so I could then go and apply the Document filter to the search that way I am only getting results for articles that have to do with chemistry.
The many filters Shelf has are located in panel on the left hand side of the dashboard and are listed below:
To understand how each filter works click here.