Reaktor/Tutorials/Reaktor as effects processor/Ensembles
Ensembles (R documents with suffix .ens) are at the top level of R and have the Reaktor icon in the Browser. Only one can be open at a time; you can't link them together. When you open an Ensemble, you'll see that it contains one or more instruments, plus the Audio In and Audio Out modules, which are only visible in the Ensemble - Structure view... and any ensemble that's currently open will close.
Ensembles can ONLY contain Instruments! All other objects must be inside instruments. To insert other object types into an ensemble, first open the structure of one of the instruments within it, then insert.