I'm leaning towards the idea that it encompasses things like special consolidator travel agent fares, fares that they give in compensation for volunteers who offload themselves in overbooked situations, or competition prizes - so instead of being processed manually, they can go in the system and a pax on one of these tickets can manage the ticket as normal via the online processes and yet still have all the restrictions (and on the e-ticket the fare type is "Special").

I don't think we'll ever see them purchasable on the website, except in heavy promotional periods (although even then the promotional fares just seem to be the same price as cheapest saver fares) or special upgrade offers.