When on, a in front of a line inserts blanks according to 'shiftwidth'. 'tabstop' or 'softtabstop' is used in other places. A will delete a 'shiftwidth' worth of space at the start of the line. When off, a 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 or spaces) depends on the 'expandtab' option. Also see |ins-expandtab|. When 'expandtab' is not set, the number of spaces is minimized by using s. This option is reset when 'paste' is set and restored when 'paste' is reset. NOTE: This option is reset when 'compatible' is set.