Variable: rectangle--mark-crutches
rectangle--mark-crutches is a buffer-local variable defined in
rect.el.gz.
Documentation
(POS . COL) to override the column to use for the mark.
Source Code
;; Defined in /usr/src/emacs/lisp/rect.el.gz
;;; Crutches to let rectangle's corners be where point can't be
;; (e.g. in the middle of a TAB, or past the EOL).
(defvar-local rectangle--mark-crutches nil
"(POS . COL) to override the column to use for the mark.")