The skeleton file Components.core intro comments states the following...
I imagine this was a previously named file type, but what type was it changed to? If I had to guess I might guess .str file. Although, ".pri" does also imply ".1st".
This file contains the definitions of assorted components that are utilized by multiple component sets. Since the component sets require that all referenced components must already be defined, and we can't control the compilation order of files with the same filename extension, these shared components are placed in a file with a ".core" extension to ensure they are processed before the various ".pri" and ".aug" files.
I imagine this was a previously named file type, but what type was it changed to? If I had to guess I might guess .str file. Although, ".pri" does also imply ".1st".