If a file named debian/package.bash-completion exists, then different actions are performed, depending on its format.
It can be a proper completion snippet, and in that case it would be installed in the completion directory, and no other actions would be performed.
It can also be a list of files, with an optionally specified name to call the completion snippet after. The file format is as follows:
my/path/to/foo-completion # this would be installed as "foo-completion" my/path/to/bar-completion baz # this would be installed as "baz"
This program is a part of bash-completion.