Where Repository Discussions Happen

Repositories on UnityFreak do not have a discussion board of their own. Conversation happens in the community, and each repository has a Discussions tab that explains where.

TL;DR — The Short Version

The repository Discussions tab opens a page that links to the community Forums and Groups. It does not create a space, move a conversation or connect the repository to a forum or group.
Forum threads are public. A group is Public, Private or Hidden. In a Private or Hidden group only approved members can read the posts and the member list, but a group is open to whoever its owner admits, not to a repository's collaborators.
For a private or internal repository the tab says so and does not offer to start a public thread. Keep that repository's code and details out of the community; use pull request comments, which follow the repository's access.
The tab appears only when the Community module is turned on for you. If you cannot see it, the community is not available to your account yet.

1. Where to talk

Forums at /forums: open threads anyone can read. Posting needs an account.
Groups at /groups: ongoing communities. Public groups are listed and anyone can join and read them. Private groups are not listed and are reached by their link; joining needs the owner's approval, and only approved members can read the posts and members. Hidden groups are not listed and show as not found to anyone who is not already a member; there is no invite flow, so a hidden group can be reached only by people who are already members.
On the change itself: pull requests carry review comments, and issues carry the linked project's public issues. Both stay with the repository and follow its access.

2. Visibility and private repositories

The page shows the repository's visibility next to the places it links to. For a public repository, mentioning it in a forum thread or a group is fine. For a private or internal repository the page shows a warning instead of a call to action, no link on it carries the repository's name, and nothing is posted or redirected for you. Any move into a public place is a step you take yourself, on the forum or group page.

A Private or Hidden group keeps its posts from people who are not approved members, but approval is the group owner's call, not the repository's: treat a group as a community, not as a stand-in for the repository's own access.

3. What is not built

A separate discussion board for each repository.
A forum category or group created automatically for a repository.
A discussion space whose readers are limited to a repository's members.

For private discussion between the people who already have access, use pull request and issue comments.