Is responsive.desktop.css for desktop & laptops design?
I'll quote from the adaptive theme documentation:


  
  Global styles always load. 
  Responsive styles load in media queries.
  


So, you don't need to use responsive.desktop.css if you can declare all your styles in global.style.css. This is most probably the case if you don't follow the "mobile first" approach. But if you do, you probably have additional styles for devices larger than tablets, and that's when responsive.desktop.css comes into play. And of course this is also for laptops. It's the screen size that defines this, not the "wording"...


  Global Stylesheets
  
  These load by default. Using a mobile first approach you would only
  place styles that are going to be used for all devices. You need to be
  ruthless about this and cut down on what is going into these files -
  its very easy to assume you will use this or that everwhere, but in
  reality you are merely falling into the familiar routine of desktop
  first theming. On your average site these files will be rather small.
  If they start getting really big, and if you find yourself overriding
  stuff later in the responsive sheets - then you know you have a
  problem and need to re-think what you are doing.


[...]


  Responsive Stylesheets
  
  Adaptivetheme provides five responsive stylesheets - one for each
  breakpoint set in your theme settings. During development (see sidebar
  Development and Production Modes) each of these files is loaded in a
  link element and places the media query inside the link element
  itself, as opposed to inside the actual file as an @media declaration.
  The media queries are those you set in the Appearance settings for
  each device type. Adaptivetheme automatically uses these so your
  styles will match to your layout breakpoints.


And do not forget about this setting in the theme settings (css):


  Mobile First or Desktop First Adaptivetheme supports both mobile first and desktop first design approaches. This setting fundementally
  alters what CSS is loaded and how it is loaded. At the most basic
  level if you select Mobile first the Smartphone portrait layout will
  load as the default, if you select Desktop first the Standard layout
  will load as the default. Many other changes take place regarding
  loading of CSS for Internet Explorer, Responsive Gpanels and Panels as
  well.