From the introduction:
A percentile-based modern role playing system
DoubleZero is a percentile-based role playing system designed to support a variety of genres, including police procedurals, detective fiction, hard-boiled fiction, caper stories, spy thrillers, techno-thrillers, and political thrillers. Unlike many games that feature magic, the paranormal, or superhuman powers, characters in DoubleZero are highly skilled men and women in a more-or-less real world.
These rules were inspired by and in emulation of a classic role playing system that has been out of print for twenty years. It follows a trend in role playing games called "retro-clones" which emulate fan-popular but out-of-print rules sets.
The DoubleZero system does not contain any setting, actual or implied. It is simply a set of game mechanics designed to emulate fictional characters in fictional situations.