Highlights all outside perimeter walls but it will not highlight any walls that are internal even if they run around the perimeter of a room. If a section of wall is internal and demarcates an enclosed space but it is not exposed to the outside environment then it will not be highlighted because the wall closes off an internal area.
This command will operate with the Select Wall tool. It can be used when either a 2D or 3D view is active. The Select Perimeter option on the Edit menu will be greyed out if the Select Wall button is not highlighted.
There is no keyboard shortcut for this command.