In Bulma, a lot of element <strong>share</strong> a set of styles. While mixins allow sharing, they repeat the CSS rules everytime they are used.
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To avoid the repetition, Bulma uses the <code>@extend</code> rule to share code. This rule tells Sass that one selector should inherit the styles of another. <ahref="https://sass-lang.com/documentation/at-rules/extend"target="_blank">Learn more about the extend rule</a>.
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Instead of creating CSS classes that might not be used to be the <strong>source</strong> of the set of styles, Bulma uses Sass <strong>placeholders</strong>:
Each of these placeholders are simply the <code>@extend</code> version of their <ahref="{{ site.url }}{{ mixins_link.path }}">corresponding mixins</a> (here for the <ahref="{{ site.url }}{{ controls_link.path }}">control mixin</a>).