debian/package.alternatives
An example of the format:
Name: editorLink: /usr/bin/editorAlternative: /usr/bin/vim.basicDependents:/usr/share/man/man1/editor.1.gz editor.1.gz /usr/share/man/man1/vim.1.gz/usr/share/man/fr/man1/editor.1.gz editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz/usr/share/man/it/man1/editor.1.gz editor.it.1.gz /usr/share/man/it/man1/vim.1.gz/usr/share/man/pl/man1/editor.1.gz editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz/usr/share/man/ru/man1/editor.1.gz editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gzPriority: 50
The fields Link, Name, Alternative, and Priority are mandatory and correspond to the update-alternatives(1) --install parameters link, name, path, and priority respectively.
The Dependents field is optional and consists of one or more lines. Each non-empty line must contain exactly 3 space separated values that match (in order) the values passed to the --slave parameter for update-alternatives(1).