The extensible SDK provides a cross-development toolchain and libraries tailored to the contents of a specific image. The extensible SDK makes it easy to add new applications and libraries to an image, modify the source for an existing component, test changes on the target hardware, and ease integration into the rest of the OpenEmbedded build system. You would use the Extensible SDK if you want a toolchain experience supplemented with the powerful set of devtool commands tailored for the Yocto Project environment.