Internal Comments option allows the organisation team to hold a discussion in private for any post. Both the members and admins can add an internal comment. This will be completely hidden from anyone except the org team. For example users will only see 👇🏽

Whereas the org teams have, 

The team can toggle between submitting an internal comment or a public comment. For easy differentiation, all internal comments are highlighted in yellow. One can reply to public comment, start a new internal comment thread or reply to existing internal comment

Even for buckets or posts with disabled comments, teams can still add an internal comment. It will be only comment box available.