What does the .in suffix in postinst.in mean?

<pusling> it is just a way to mark that this file will somehow be processed into real file

Consider:

x61:~% ls /tmp/dwm-4.7/debian/*.in
/tmp/dwm-4.7/debian/postinst.in  /tmp/dwm-4.7/debian/prerm.in

Why postinst.in?

x61:~% head /tmp/dwm-4.7/debian/postinst.in
#!/bin/sh

set -e

ALTERNATIVES="@ALTERNATIVES@"

case "${1}" in
		configure)
				update-alternatives --quiet --install /usr/bin/dwm dwm /usr/bin/dwm.default 100

x61:~% egrep postinst /tmp/dwm-4.7/debian/rules
		rm -f debian/postinst debian/prerm
		sed -e 's/@ALTERNATIVES@/web/' debian/postinst.in > debian/postinst; \

See how postinst is generated?