It is the same helper. From the documentation
When using the asset pipeline, paths to assets must be re-written and sass-rails provides -url and -path helpers (hyphenated in Sass, underscored in Ruby) for the following asset classes: image, font, video, audio, JavaScript and stylesheet.
-url
is used for sass, _url
is used for Ruby.
It is convention in css and sass to use hypen-seperated words for functions; whereas in Ruby, it is convention to use snakecase.
Both functions should have the same behaviour and hence generate the paths that point to the same asset directory. If they do not, then you are probably experiencing a bug similar to: sass-rails asset pipeline: generating image paths incorrectly; `url(/images/blah.png)` instead of `url(/assets/blah.png)`