Why do some Verizon iPhones have sim card slots?

So why is it that some verizon iphones have sim card slots and some don't? I'm talking about the visible one externally on the side of the iphone. I have seen some Verizon 4S with no sim card slot & some with a sim card slot. Why is that? Are the ones with the slot jailbroken AT&T iphones?

The Verizon iPhone 4S as well as the Sprint iPhone 4S both have sim slots. All of them. That's for international use and roaming. Still can't use them on AT&T because its still a CDMA phone. You must be seeing iPhone 4s (not the 4S, the iPhone 4). They don't have SLIM trays whatsoever unless they're for AT&T. Places like Verizon will sometimes put an iPhone 4 on a 4S stand if they run out. Walmart does the same thing with iPads. I still see first generation iPads on the Walmart stands that say the New iPad (which is the iPad 4).

All Verizon iPhone 4S has SIM slot and Verizon iPhone 4 doesn't have a SIM slot.