Users can share user-ids between systems using a password/login scheme. Anyone can use any UserName they like (as long as it is a valid LinkPattern). The User-id (a number, 1001 or greater) is what UseModWiki uses to distinguish users, and store their individual preferences. This user-id is stored in a cookie on your system, along with a random number to make it non-trivial to get another user's information. You can see the user-ID as the link title of your name on RecentChanges (as a tooltip message on MSIE and Netscape).
The new password/login scheme allows people to share a *single* user-ID between multiple systems (like work/home systems). One could enter a password on the home system, and then go to work and enter the user-id and the same password. UseModWiki will then send the home cookie to your work system, and the two systems will share a single user-id.
Right now there isn't much need for this feature, as there are relatively few customizable settings. You can simply have multiple user-ids and set each one up separately.