REM is a font-size unit based on a default value—typically 16 pixels—that allows you to design type scales relative to that base value. This base value can even be customized per Breakpoint, making all text layers using REM responsive out of the box. When using Layout Templates, this gives you a single place to define both your Breakpoints and base sizes per Breakpoint. You can then tweak either of them without having to redo any of your Text Styles.