TD-12 Decoupler
The TD-12 is listed in Engineering 101; the tutorial describes stack decouplers as enabling multiple stages and separating used rocket parts.
How it works
The technology tree lists the TD-12 Decoupler in Engineering 101, which follows Start and is listed at 5 Science. The Science tutorial says a stack decoupler allows multiple stages and describes using a decoupler to separate fuel tanks and engines for a smoother landing. Its worked example names a TR-18A Stack Decoupler, so that specific procedure is not a model-specific specification for the TD-12.
When it helps
If your design uses multiple stages, the tutorial says a stack decoupler allows them and can separate used fuel containers during ascent.
Read the supporting references
Check the edition and source version for patch-specific details. Guide methodology.