This policy setting specifies the order in which Windows Installer searches for installation files.
If you disable or do not configure this policy setting, by default, the Windows Installer searches the network first, then removable media (floppy drive, CD-ROM, or DVD), and finally, the Internet (URL).
If you enable this policy setting, you can change the search order by specifying the letters representing each file source in the order that you want Windows Installer to search:
-- "n" represents the network;
-- "m" represents media;
-- "u" represents URL, or the Internet.
To exclude a file source, omit or delete the letter representing that source type.
n = network, m = media (CD), u = URL
A few valid examples: nmu, n, nu, mn