If you see the SDK content
there are framework, toolchain, device image …
So I would say SDK is a package which contains all the tools you need for your development.
e.g. you need toolchain to compile your application
you need device image to upgrade the image of your module…
For application framework, you can have a look on the content in
It contains the source code of the application framework. You can compiled the application framework by the toolchain you downloaded and generate the legato.cwe so that you can download to your module.