Re: Unlocked Lumia Icon
What you really need to look into is what bands the phone works on and what bands the carrier you want to go with works on. Theoretically, the 930 should work on AT&T's and T-Mobile's LTE networks. However, it will depend on the area.
Here is a link where you can look up your area to determine what bands are being used.
Spectrum Dashboard - Reboot.FCC.gov
Generally speaking CDMA phones are considered locked. For example, the Icon is "locked" because of CDMA. CDMA requires a unique key that is tied to the carrier, in this case Verizon. Once your contract is up, Verizon can "unlock" the phone, but the problem is that if you move to another carrier such as Sprint, that carrier has to be able to support the phone. The CDMA key is part of the firmware and if Sprint doesn't have the firmware for the Icon you can't use it. Of course you can use the GSM/HSPA+ network on AT&T or T-Mobile, but neither of those two carriers use the same LTE bands as Verizon.
Unlocked means it can be used on any carrier regardless of branding. Unlocked usually refers to GSM phones. GSM does not require a unique key to operate on a carrier's network, it just needs an activated SIM for that network. As long as the phone supports the frequency bands of the network, you will be able to use it.
LTE is a direct descendent of GSM; so as long as your phone supports the LTE bands of the carrier you choose, then all you need is an activated SIM for that LTE network.
A good example of a branded versus unbranded phone would be the Galaxy Nexus from a couple years ago. There were essentially three versions. The unbranded version that you could get directly from Google, this was unlocked and could run on AT&Ts an T-Mobiles LTE networks here in the US and abroad. Then you had the Verizon branded version that was CDMA locked, but could also run on AT&T's and T-Mobile's GSM networks as well as abroad, and you had the Sprint branded, again CDMA locked to Sprint. Like Laura said though, the unbranded version got its updates directly from the OS maker, in this case Google, however, the Verizon and Sprint branded versions got their updates from Verizon and Sprint respectively.
This is why manufacturers and OS makers prefer unbranded and unlocked phones, because it simplifies manufacturing and OS development and update roll outs. But the carriers, US carriers in particular, don't like this because people can switch carriers whenever they want.