Callouts - Obsidian Help mentions that the tip callout has aliases hint and important.
While working on some theme, I noticed that the base app.css has a CSS definition for important, separate from tip/hint. This includes having its own dedicated --callout-important that can be styled in a different colour than tip/hint (--callout-tip) along with a potentially different icon.
They do, in the default theme, share the same styling, but that’s not guaranteed when using other themes (nor maybe the default theme in the future).