A permanent portal has an opening in Sight and an opening some place else (on another plane, in the
natural world, or in some pocket dimension) that are always available for use. Every time you step through a permanent portal, you know where you’re going to end up. There are no surprises. (Unless, of course, you have no idea where a particular portal connects to.)
Permanent portals tend to connect relatively safe and stable locations. such as gate towns. to Sigil. Many permanent portals are known by someone in Sigil and the information pertaining to them tends to be valuable currency within the City of Doors.