subtle is a manual tiling window manager with a rather uncommon approach of tiling: Instead of relying on predefined layouts, subtle divides the screen into a grid with customizeable slots (called gravities). For better understanding, the default config uses a 3x3 grid and maps each gravity to one key of the numpad. With those keys, windows can be moved directly to the desired gravity - the same can be done with tagging rules in the config.
Another unique concept is the strict tagging: Unlike other tiling window managers, subtle doesn't allow weak tagging and always maps windows to virtual desktops (called views) with matching tags, regardless of the current active view.