As we know the new HTC One has been available globally in numerous variants, which based on the local regulations and the network infrastructure. In some countries that already have 4G network, for instance North America as well as some countries in Europe and Asia, the phone is available in LTE (UL variants), while in countries that don’t support 4G network, they only offered GSM device only (U variants).
In the U.S., UL variant released on AT&T and T-Mobile branded HTC One. The best thing is you can install custom ROMs, kernels or any mods that were originally made for U variant (GSM only devices) to be used in UL variants (GSM/LTE). Explanation of the One’s variants can be seen in the picture below, which originates from androidrevolution.nl.
It’s very beneficial for users and developers, for HTC One devs now they don’t need to build and release a different ROMs for each variant, just build one ROMs and use it on many variants (UL or U devices). For users, now they have a lot of selection of mods to be used on their devices.
This is confirmed by the AT&T branded user who utilize CWM recovery that was originally developed for the International version. For those who have a Sprint branded phone, it seems you can not get the same freedom as the others since your devices is WL variants.