RUU is Rom Update Utility. It's a complete system wipe flash to stock update used when a update patch is not possible. It's keyed to a particular model and checks to make sure before it installs. Some models have never received a full update (US wifi for example) so no RUU ( in public ) exist for them. HTC does have a universal RUU that they use if you return a bricked unit, but we don't have a copy of that.