When on, a <Tab> in front of a line inserts blanks according to 'shiftwidth'. 'tabstop' or 'softtabstop' is used in other places. A <BS> will delete a 'shiftwidth' worth of space at the start of the line. When off, a <Tab> always inserts blanks according to 'tabstop' or 'softtabstop'. 'shiftwidth' is only used for shifting text left or right shift-left-right. What gets inserted (a <Tab> or spaces) depends on the 'expandtab' option. Also see ins-expandtab. When 'expandtab' is not set, the number of spaces is minimized by using <Tab>s. This option is reset when 'paste' is set and restored when 'paste' is reset. NOTE: This option is reset when 'compatible' is set.

(default off)