The ATTN Portal System can transport between various locations and dimensions, using a 20 digit number to determine target destinations. Of note, the mirror within the Black Rose Inn is specifically set to draw in heroes from other worlds to attract help for their ongoing difficulties, and as such, people may inadvertently wander in. Don't panic.
In the absence of a specific code being entered, or other modification, a person who contacts the transference medium (mirror) is transported to the transference medium through which they reached this destination (often a doorway, mirror, or other surface through which one cannot see.) This is convenient, as it allows lost travellers to return home with relative ease.
However, most people can also use the keypad to give a 'Home' number that they can share with others, so that they and others can return to their home dimension for visitation or adventuring purposes.
The Keypad is a holographic interface that appears within the mirror if it is active, and allows people to change the configuration of the mirror – opening portals to other locations, for example, or controlling how it responds to the user or other guests. People with Trusted Access in a particular location can thus lock it against entry, block or whitelist specific individuals, or ban specific individuals (transporting them to a random hostile blocked dimension when they attempt to enter!)