A register can contain a rectangle instead of linear text. The rectangle is represented as a list of strings. See section Rectangles, for basic information on how to specify a rectangle in the buffer.
copy-region-to-rectangle). With numeric argument, delete it as well.
The C-x r i r command inserts a text string if the register contains one, and inserts a rectangle if the register contains one.
See also the command
sort-columns, which you can think of
as sorting a rectangle. See section Sorting Text.