WRT to "unknown" - if one of the parents is a cultivar that you know, but it is not registered with the AHS, the AHS requires you to list that parent as "unknown". So unknown does not necessarily mean unknown. I have used a few cultivars from Olallie in Vermont that perform excellently in Zones 4 and 5 and throw great and reliable kids, but Olallie doesn't register many of their cultivars. Hopefully, if the hybridizer is using such a plant as a parent, they will explain what it is on their website, so buyers have a better idea what they're getting.
I don't know why people use "sdlg x sdlg" if they know the parents - it's just as easy to list "(cultivar A x cultivar B) x (cultivar C x cultivar D)" as the parents and gives buyers valuable info. Sometimes seedling markers get lost, in which case they're stuck with "sdlg x sdlg" but even there, it's helpful if the hybridizer will at least make an educated guess on their website - they often have a pretty good idea, but aren't totally sure and are honest enough not to register it using a questionable parent.
There are some hybridizers who do thousands of crosses and do not keep track of the parents. Saves them a lot of time and effort, but it means buyers either have to accept getting a pig in a poke, or will wait to hear from others to see if the intro turns out to be a good performer or good parent. Most of us would like as much information as we can get before we shell out the big bucks!
