There is only one file type that uses the .LTZ extension:
Compressed Poser Light Set File
Compressed lighting information file used by Poser, a 3D character modeling and animation program; includes light data, such as spotlights and distant lights; Poser 6 and later may also contain point lights and image-based lights.
Uncompressed Poser light set files use the .LT2 extension.
NOTE: Poser was originally developed by Smith Micro, but was sold to Bondware in June 2019.
Updated: September 20, 2020