Sliding and overturning checks are done for the global X and Z axes. Here are two simple images to illustrate how this works.
For overturning you can think a rectangle being drawn so that it encompasses the entire slab. Overturning checks are then done about each axis separately considering the extreme edges of the slab.
For example, on the X axis we find the largest and smallest X coordinate location on the slab and do a summation of overturning moments and a summation of resisting moments. These summations, as well as the Factors of Safety, are presented in the Safety Factors spreadsheet.
Additionally, sliding checks are done. The coefficient of friction for the soil is set in the Solution tab of Global Parameters, allowing us to give a summation of sliding forces and a summation of resisting forces in the global directions. These summations, as well as the Factors of Safety, are presented in the Safety Factors spreadsheet.