Not all programs use UNC names in their I/O requests. Some applications use WNet APIs (which are the Win32 network APIs). The Multi-Provider Router (MPR) was created to support these applications.
MPR is very much like MUP. This layer receives WNet commands, determines the appropriate redirector, and passes the command to that redirector. Because different network vendors will use different interfaces for communicating with their redirector, there is a series of provider DLLs between the MPR and the redirectors. The provider DLLs expose a standard interface so that MPR can communicate with them, and they know how to take the request from MPR and communicate it to their corresponding redirector.
The provider dynamic-libraries ( DLL) are supplied by the network vendor that wrote the redirector and should be installed automatically when the redirector is installed.