backward-delete-char is an alias for `delete-backward-char' in `subr.el'.

(backward-delete-char N &optional KILLFLAG)

Delete the previous N characters (following if N is negative).
If Transient Mark mode is enabled, the mark is active, and N is 1,
delete the text in the region and deactivate the mark instead.
To disable this, set option `delete-active-region' to nil.

Optional second arg KILLFLAG, if non-nil, means to kill (save in
kill ring) instead of delete. Interactively, N is the prefix
arg, and KILLFLAG is set if N is explicitly specified.

In Overwrite mode, single character backward deletion may replace
tabs with spaces so as to back over columns, unless point is at
the end of the line.