In 2D game engines, every object on screen has an "active" or "inactive" state. The is used to maintain a sparse set of active objects. Instead of iterating over all 10,000 objects every frame, the engine maintains an array of indices where is_alive = 1 .
A is the purest form of a two-state index. For each possible value (state), the database creates a bitmap (a string of bits). The length of the bitmap equals the number of rows. For the above table: index of 2 states