A region is a division of the area map in exploration scenarios. Depending on the distance covered by the scenario, an individual region may represent a room, wing, building, or an entire forest or similar piece of terrain that may take multiple days to cross.
Each region is given a descriptive name and a set of exits that function as connections to other regions in the map. In this manner, the area map is more like a set of connected nodes rather than a subdivided location as used in combat battle maps.
Each entity occupies a region. Some Challenges may, at the DM's discretion, occupy multiple regions; this is most common with Environs such as weather conditions that could even affect the entire area map. Obstacles are placed on exits and are considered to occupy the regions on both sides of the exit.
Regions can have various properties that apply special rules. For illumination rules, see lit and dark. For swimming rules, see the section in the Player's Guide.
Region exits may be Hidden or blocked by an Obstacle. If an exit is not blocked and there are no issues with illumination or other visibility-blocking effects, PCs can see beyond the exit into the next region. Visibility only extends to immediately adjacent regions; PCs cannot see across multiple exits in a chain. Creatures and Seekers are revealed from this visibility (unless Hidden), but Environs, Obstacles, and Traps are not, nor are any of the region's other exits revealed (if the region has not yet been visited). Traps typically remain unrevealed until they act (usually through an Interrupt) or are found by a Search.
For more information, see Player's Guide Chapter 5: Exploration.