system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] What would the ramifications of the Tau discovering a complete STC be?

Would they be able to decode it's contents? 

How would it affect their sanding in the galaxy?

Would the Impirium mount another crusade?
gpt
Let's assume that the Tau find a fully functional STC. Like, the kind of thing that every single person in the Mechanicus has wet dreams about. Let's also assume that the Imperium catches wind of this, somehow. 

The hammer is going to come down so fucking hard. Not only is there an STC up for grabs, but it is in the hands of filthy Xenos. You are going to see, basically, a mini crusade; multiple chapters, huge numbers of Imperial Guard and Navy, full backing from both the High Lords of Terra and the Mechanicus. You are going to watch a clusterfuck of galactic proportions open up. The Imperium goes crazy over RUMORS of STCs; how do you think they are going to react if they KNOW that there is one?

Now, right away the Tau are going to know that this thing is special. They might even figure out how to get it to start making stuff if they tinker with it enough, especially since the STC is designed to be largely automated. They start pumping out Baneblades and smaller Titans, then retrofitting Tau tech into them. They might even find that they can create "Heavy Battle Tanks" (Baneblades are considered "light" tanks according to a partial STC) which make the Imperial Guard's armor look like playthings. 

By the time the Imperial STC reclamation crusade shows up the Tau have practically doubled their military power. The war quickly turns into a meat-grinder as billions of soldiers clash with the very weapons they have come to rely on, bastardized by Xenos tech. It becomes a war of attrition as Imperial forces are thrown into the fray en-masse. 

Titan legions are mustered, along with armored divisions across the Imperium. The largest grouping of Imperial armor ever starts coming together. Both sides watch in horror as their war-behemoths lay waste to entire planets trying to kill each other. 

The closer to the center of the Tau Empire they push, the more fearsome their foe becomes. Border worlds are quickly quelled, but more central worlds do not go down easily, especially when bolstered by Dark Age tech. 

The war quickly grabs the attention of the Eldar, who also want the STC if for no other reason than to stop anyone else from having it, especially the Imperium. Eldar fleets start pouring in, and a three-way fight starts up. The Orks catch wind that there's a giant brawl going down and they start flocking to the Tau Empire like ants to a bowl of sugar. One of the largest WAAAAAGHs! ever seen materializes. The Tyranids are drawn to the incredible grouping of biomass. Chaos, not wanting to be left out, shows up with another Black Crusade, hoping to take the STC for themselves.

After years of agonizingly brutal combat the war parties start making their way into the central system of the Tau. On just a handful of planets we see the largest war host ever amassed by the Imperium, a craftworld's worth of Eldar, a quickly growing WAAAAGH! that threatens to make the Octarius War look like childsplay, a full tendril of Hive Fleet Leviathan, a new Black Crusade, and the entirety of the Tau military, backed against a wall, fighting harder than they ever have. 

The Necrons take a nap.