It varies based on the reticle. The .308 models are designed for the 7.62mm M80 out to 1200 meters. The .50 reticle is designed for the .50BMG round out to 1800 meters (no specific round model provided). The M2 model is for the Browning M2 .50BMG machine gun, and the M240 model is for the M240 machine gun. Non-caliber specific models are not pre-calibrated, you can use them with any caliber and bullet weight you would like.
I would say whatever bullet you will be shooting most of time. I would zero it with Hornady 75 grain HPBT V-Max. If you shoot a lighter weight bullet, it will shoot a little higher.