In Reaktor, there are 3 different levels to work in. You have your instrument level. Then you have the interior parts that make up that instrument which are called cells. Inside those cells you have the CORE structure of the cell which are the working parts of that cell. This is the lowest level to make modifications.

Chris Ozone

The CORE is the lowest of three levels that help to make up the overall instruments/devices used in the Reaktor Program.

Travis B