It's virtual desktops but with a fancier name, and a minor bonus feature where specific windows can appear on multiple virtual desktops at once. DWM uses bitma

It's virtual desktops but with a fancier name, and a minor bonus feature where specific windows can appear on multiple virtual desktops at once. DWM uses bitmasks to keep track of what window is on what virtual desktop and bitmasks can have multiple bits set, so windows can be on multiple desktops/"tags".