Active regions originally come from Zmacs, the Lisp Machine editor. Put simply, you can only operate on a region that is highlighted. The region is deactivated after each command that is executed, except that motion commands do not change whether the region is active or not. It will remain highlighted until some non-motion comand is executed. When zmacs-regions is t , there is no distinction between the primary X selection and the active region selected by point and the mark.
Errors also remove highlighting from a region.
Certain commands cause the region to be in its active state. It is reset to nil after each user command is executed. Only a small number of commands should ever do this. Go to the previous , next section.