The S1-200 can be used in horizontal position as long as it is protected from dirt or mud. Pumping water with higher particle concentrations than indicated here will reduce pump lifetime and void the warranty.
For more information you can read the article on particle concentration at the following link:
If the pump is used in a horizontal position, dry run protection cannot be used because the dry run sensor needs to be in a vertical position. It may be installed alternatively (ie pole or stick).