Objectives: The designer, client and developer can understand how the website will appear across different devices.
Outputs: Designs, so as to provide as full an understanding as needed in terms of the site's responsive behaviour.
This should include:
Mobile appearance
Tablet appearance
Desktop appearance
How the site will respond, upwards of 1920px
Information on how the site will scale up and down in between these points (fixed widths, fluid widths etc).
This can be accompanied by a description of how the site should respond at different breakpoints, and if the website is suitably formulaic in terms of it's page structure - may not require all pages to be individually designed.