Building Exploration tokens (sometimes also known as Door tokens), are placed on Buildings during the setup for a Mission. They mark that the building has not yet been explored.
Building
A Building in Chronicle X consists of a number of Mission Map Squares surrounded by a wall (drawn on the Map Tiles). When setting up the Mission Map, each Building receives a Building Exploration Token.
The first time a Hero or Alien Unit enters a Building where there is a Building Exploration token, the token is removed and a Building Exploration Card is drawn and resolved. The Hero or Alien Unit doesn't need to step on the specific Square that the token was on; entering any Square inside the Building triggers the exploration event.)
If adjacent Map Tiles have building walls meeting along the edges, and one of the walls has an opening, it counts as there being an opening in the wall on the adjacent Map Tile as well, even if there isn't one drawn on the Map Tile.
REFERENCE: FAQ ANSWER FROM ARCHON: Q: How can buildings be joined? How are rooms added? ANSWER: As long as there is at least one opening on the touching walls, those can be considered as joined. An opening on one tile will negate a solid wall on another tile (as long as they are touching).