Thanks, I have seen (& certainly appreciate!) all the detail. I was hoping for a (very) high level over view. The best I've seen is from the current (4.3.1) Skinning.pdf on page 14:

A skin package can contain a global attributes specification named “skin.xml” ( or “container.xml” for containers ) which applies to all skin files in the package. In addition, you can also override the global skin attribute specification with a skin specific attribute specification by providing a “skinfilename.xml” file. The skin uploader will merge the skin attributes with the HTML presentation file to create an ASCX skin file.

Please note there is a one to one correspondence of skin object declarations in your skin file ( ie. [MENU] ) with the attribute specification in the XML file.

I guess my net take-home message is that XML files are useful only when doing the HTML style skins (versus default.ascx) and is how HTML skinners can set CSS value defaults.

Another thread of some interest is at:

http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/109/threadid/37255/scope/posts/Default.aspx

where John Mitchell writes:

The skin.xml file is for setting default attributes on controls, but if the control in question also uses css classes for it's style (like the Solpart menu) then those will be the actual values applied in the browser when the html is finally rendered there.

Thanks All!