The only special exception for this "global" locale is that any other locale can reference it via a relation field of a localized content type.

Currently, there are two issues with locales and relations:

1) non-localized content types CANNOT have a relation to a document of ANY locale of a localized content type (covered: https://feedback.strapi.io/feature-requests/p/show-all-locales-in-a-non-localized-relation-field)

2) localized content types CANNOT have a relation to a document of a non-localized content type in a locale DIFFERENT from the default one (covered here)