what is difference between unlocked iPhones?

What I'd like to know is exactly what is the difference if I buy an unlocked iPhone for full price at the Apple store compared to paying full price at Verizon? (They said the iPhone would be unlocked when paying full price or paying it off over time, which is supposedly true for all phones now.) Also, starting with the 6S Upgrade payment program, is this the only way to buy unlocked from Apple or do they still sell unlocked at full price? Is a sim-free unlocked iPhone different in bands/carriers compatibility compared to the other carrier versions which are unlocked?

Looking at the online table of models and carriers, there seems to be some differences so is a sim-free iPhone 6S the only version 100% unlocked for all bands/carriers everywhere? is there such a model? or are there always some differences since some are still CDMA for voice services? and those models do not cover all bands on other systems?

Would be nice if someone from Apple could give some definitive answers.