Usually you can leave Qiso to handle all of the mathematics for you, but in some circumstances you might need to know which tile is positioned at a specific screen pixel for example to handle player interaction outside of the engine. In which case Qiso provides a number of functions for converting between screen, cartesian, and isometric co-ordinates.
Tile co-ordinates refer to the position of a tile within the raw tilemap data. I.e. in a 4x4 grid of tiles, the x,y of the second tile on the first row would be 2,1.
Cartesian co-ordinates refer to the top-down co-ordinate of something within the map. For example in a grid of 100px wide tiles, the cartesian x of a player standing on the second tile could be anything between 101 and 200.
Isometric co-ordinates refer to the co-ordinate of something rendered in the isometric world.
Screen co-ordinates refer to the traditional, flat x,y screen co-ordinate.