in HeuristicLab we use the convention that the start index of a range is
inclusive and the end index is exclusive.
The training partition contains rows [0..12[ and the test partition contains
rows [12..25[. This represents a 50/50 split of the available rows which is
also used as a default when loading new data into a symbolic regression
problem. However, you can change the start and end indexes as you wish.
Best,
Gabriel