/sys/uid.sys

From WiiBrew

uid.sys is a list of TitleID-UID pairs. UIDs are used for ownership on the Wii filesystem.

typedef struct
{
	u64 title_id;
	u16 padding;
	u16 uid;
}

The first entry in the file is always Title 1-2, with UID 0x1000. UIDs iterate from that number. It may not be necessary for the first entry to be 1-2/0x1000, but it's probably a good idea to keep it that way :)

On the filesystem, ownership is normally used to gives titles access to their "data" directories.