HaoZhe (Boy !! its hard to rememeber the name), Its the OS under which you developed and ran the software. (You could write the document in which ever OS you want.. but it's the software that counts) Dushy
Before you submit your application, make sure that you have tested it on at least Windows and Linux. Test on as many platforms as you can before you submit. You want to be sure that your file paths, etc. are going to work in all situations.