See the manual for differences between these modes. Briefly, bare mode stores files as they are, so they can be directly hardlinked, bare-user uses extended attributes to store ownership and xattr information, allowing non-root operations, bare-user-only does not store ownership information, and archive stores files compressed, to be served over the network.
This collection ID must be persistent and globally unique. It is formatted as a reverse DNS name (like a D-Bus interface). It must be set to a reverse DNS domain under your control.
This option may be omitted (the default) to leave peer to peer distribution unsupported for the repository. A collection ID may be added to an existing repository in future to enable peer to peer distribution from that point onwards.
If the collection ID is changed for the repository in future, peer to peer distribution of refs from the repository will break for all peers who do not update their remote configuration to the new collection ID.