copy-rectangle-to-register is an interactive compiled Lisp function in `register.el'.

It is bound to C-x r r.

(copy-rectangle-to-register REGISTER START END &optional DELETE-FLAG)

Copy rectangular region into register REGISTER.
With prefix arg, delete as well.
To insert this register in the buffer, use C-x r g.

Called from a program, takes four args: REGISTER, START, END and DELETE-FLAG.
START and END are buffer positions giving two corners of rectangle.

Interactively, reads the register using `register-read-with-preview'.