The extensible SDK provides a cross-development toolchain and
libraries tailored to the contents of a specific image.
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.
The installed extensible SDK consists of several files and
directories.
Basically, it contains an SDK environment setup script, some
configuration files, an internal build system, and the
devtool
functionality.