Programmable per-key remapping, layered modifiers that you can toggle per app or workflow (ie. quick language layout swaps, temporary low-level remaps for games that don't support remapping, Blender/Photoshop/editor-specific remaps for personal ergonomics, etc.). Tap-hold functions to send different keycodes or modifiers when you tap vs. hold down a key, often used for home-row mods that move modifier keys into the home row when you hold, say, an HJKL key.
