Sass partials - to underscore or not to underscore?

In the Challenge, Sass: Split Your Styles into Smaller Chunks with Partials, the challenge states:
_**Names for partials start with the underscore ( _ ) character, which tells Sass it is a small segment of CSS and not to convert it into a CSS file.

Am I right in thinking that:

  1. with the underscore, the partial file being imported is a .css file not a .scss file?
  2. if the partial is a .scss file, the underscore is not required? I ask that because I have imported .scss partials before without them being prefixed by an underscore.