The only items that can perform a function and satisfy a need. This should not be confused with "single source." An item can be proprietary and yet available from more than one source. For example, if you need a camera lens for a Nikon camera, the only lens that will fit is a Nikon lens, thus, this lens is "proprietary." However, the Nikon lens is available from more than one source, thus, it is not single source.