Hi Paul,
Yes, you're right as you can find in our Developers Guide -
Managing sites.
The difference is:
Site CSS stylesheet - Default CSS stylesheet used for all pages on the site unless they override the value with their own CSS stylesheet.
Editor CSS stylesheet - CSS stylesheet used for the WYSIWYG editor content.
So you are able to edit pages with a different style (e.g. mobile) in the WYSIWYG editor in CMS Desk while the site is running a main style (e.g. desktop).
Best regards,
Martin Danko