In a perfect world, all smartphones would be able to connect to any wireless carrier’s network. But we do not live in a perfect world, and as such most smartphones are tied to either CDMA or GSM networks, thereby narrowing a user’s choice of wireless carrier. The exception is when a smartphone is unlocked to support both types of networks.

Source: Hot Hardware – Samsung And Best Buy Begin Taking Preorders For Unlocked Galaxy S8 and S8+ Handsets