PRT is a format used by PTC software (Pro/Engineer, Pro/E Wildfire, Creo Parametric) since 1987. A number of other programs also have different-format files under the PRT name, so things can get confusing.
Kubotek programs will read these files from version 16 (Pro/E) on.
File names may have numeric revision numbers (.1, .2, etc.) tagged onto them.