Bourbon’s margin mixin is just a convenience function that allows using null to skip specific margins while specifying the others. Note This is different than specifying 0 in a typical margin shorthand declaration:
margin: 0 10px 3em 20vh
// equivalent to the following
margin-top: 0;
margin-right: 10px;
margin-bottom: 3em;
margin-left: 20vh
@include margin(null 10px 3em 20vh)
// equivalent to the following
// note the lack of a specified margin-top
margin-right: 10px;
margin-bottom: 3em;
margin-left: 20vh