Borders can be assigned to sprites via the Sprite Editor, and the borders can be queried via the Sprite.borders API. However, I cannot find documentation on what purpose sprite borders serve. Here are two pages in my local copy of the documentation where I might like to see a few sentences explaining what Sprite borders are, and what purposes they typically serve in a game:

file:///Applications/Unity/Unity.app/Contents/Documentation/html/en/Manual/SpriteEditor.html

file:///Applications/Unity/Unity.app/Contents/Documentation/html/en/ScriptReference/Sprite.html

Also in the Scripting API documentation for Sprite.rect, "size" is a broken link. Was the API for Sprite.size deprecated? Here is where I see this in my local copy of the documentation:
file:///Applications/Unity/Unity.app/Contents/Documentation/html/en/ScriptReference/Sprite-rect.html