From The TimeSplitters Wiki

Tilesets are Mapmaker tools used for adding the feel of the map. Each game has a few tilesets. Changing the Tileset can greatly affect the feel of the level, for example, in TimeSplitters 2 an Industrial Tile map would look like an old, abandonned factory, while changing it to the Alien Tile would give a clean, space look. Additionally, each tile set has 1 or 2 rooms that completely change their looks depending on the set.

The original author of Tileset does not have any further information on this article. The author may return to finish the article when information has been gathered, however they welcome anyone to expand on this article.