Module
Every tactical map you play through is made from an amount of map modules. Each module is either 10x10 tiles, or 20x20 tiles, and may either be 2 or 4 units in height. By randomly merging these modules together, the game engine can produce an impressive amount of unique tactical maps, making every single encounter different to the last.
Small maps (for small and medium scouts) are 3x3 tilesets, and thus have a total of 900 tiles (3x3x10x10). Large maps (all larger UFOs and cities) are 5x5 tilesets, and thus have a total of 2,500 tiles (5x5x10x10).