Module
Every tactical map you play through is made from an amount of map modules. Each module is either 10x10 tiles or 20x20 tiles; underground/base tilesets are two levels high, and outdoor tilesets are four levels high. By randomly assembling these modules together, the game engine can produce an impressive amount of unique tactical maps, making every single encounter different to the last.
A 4 module x 4 module arrangement (1,600 tiles: 4x4x10x10) is used when encountering Small and Medium Scout UFOs. All larger UFOs and terror missions are placed a 5x5 arrangement (2,500 tiles). The surface level of Cydonia uses a 5x5 tileset, and the underground level uses a 6x6 tileset (3,600 tiles).