This is a News backend, implemented using NNTP (NNRP?).
NNTP is really a server protocol, at least thats what the guy who wrote the RFC wants, so this is pretty limited. But it provides basic news reading and posting capabilities.
Searching is supported based on a local cache of messages and a normal Evolution/Camel.Search#Camel.FolderSearch driver. All messages are cached based on the Message-Id in a global cache for all folders on the server (NNTP requires all Message-IDs to be unique).
Posting is achieved by appending to a given folder, with the appropriate Newsgroups header set for cross-posting.
Messages are stored and retrieved in the same format as RFC822 messages - which already contain newsgroup extensions anyway, for example the References header.
Apart from using XOVER(view), I dont think any other extensions are used, although password authentication is supported.
Internally, UIDs are a combination of the message index, which is used to retrieve the message from the server, and the Message-ID, which is used to cache it.
Not sure what else to say here ...