When on, the ":substitute" flag 'g' is default on. This means that all matches in a line are substituted instead of one. When a 'g' flag is given to a ":substitute" command, this will toggle the substitution of all or one match. See complex-change.

    command         'gdefault' on   'gdefault' off
    :s///             subst. all      subst. one
    :s///g            subst. one      subst. all
    :s///gg           subst. all      subst. one

NOTE: This option is reset when 'compatible' is set. DEPRECATED: Setting this option may break plugins that are not aware of this option. Also, many users get confused that adding the /g flag has the opposite effect of that it normally does. This option is not used in Vim9 script.

(default off)