As a student you're on CPT and OPT. And after you graduate, OPT.
Is there a huge difference?
You can't work for your own company while on CPT and you can while on your first year of OPT and with some additional restrictions/requirements you also can work for your own company while on STEM OPT.
Can you please tell us more about the STEM OPT restrictions/requirements? I am exactly in that position right now!