If you buy one of the compatible iPad models from a carrier retail store, it might come with a carrier-specific SIM that only works on that carrier's network. Check with your carrier for details.
When you activate a cellular data plan with certain carriers, your Apple SIM might be dedicated to that carrier's network. A dedicated SIM means you can't activate it on another carrier's network. For example, if you activate an AT&T plan on your iPad, your Apple SIM will be dedicated to AT&T's network. Your iPad remains unlocked, so you can buy another Apple SIM.