From Rare Gaming Dump

uid.sys is a file present on the standard Wii NAND filesystem, containing a list of titles which have been installed onto the system and their filesystem UIDs. While its intended use is to serve as a UID lookup list, it can also be used to determine all titles which were at one point installed onto a Wii system, even if they have since been deleted. Since uid.sys tracks both NAND titles and inserted disc titles, it can be used to determine a list of every disc ever inserted into a Wii system.