A Standard Template Construct, almost always abbreviated to STC, is a form of ancient technology dating from humanity's long-lost golden age, when mankind spread across the stars armed with knowledge it has since forgotten. Each STC was a self-contained database designed to let colonists on distant worlds build anything they required, from tools and weapons to vehicles and buildings, using whatever materials were at hand. In their day these systems could guide the construction of almost any device, ensuring that even the most isolated settlement could raise a functioning civilisation.
Over the millennia of war and collapse that followed, the vast majority of STC systems were destroyed or lost, and the knowledge to create new ones vanished entirely. What survives are fragments, partial templates and copies of copies, upon which nearly all of the Imperium's technology is now based. The recovery of even a single new STC fragment is an event of galactic importance.
The Adeptus Mechanicus revere these relics above almost all else, and their tech-priests will go to any length to recover a lost STC, treating each as a sacred text rather than mere data. The designs preserved on surviving templates are reproduced without deviation across countless forge worlds, which is why the same patterns of tank, gun, and machine appear unchanged across thousands of years and half a galaxy.