Differences between Sheet Sets and saved Publish lists?

There seem to be two types of saved lists for collections of drawings and layouts.  I am trying to figure out the differences between.

  1. FILE/PUBLISH command, and it saves the lists as a .dsd, "Drawing Set Description file".
  2. FILE/OPEN SHEET SET command, and it uses a .dst file. "Sheet Set File".
Why the two types?

-Joe Dunfee