Are these tones, characters, or tones + characters + words or something else? E.g. if I see 学习 for the first time in a session and I already knew 学, do I get one item added for 学习, or four items added (学习, 学习 tone, 习, 习 tone).

Also how well do you need to know a character for it to count as known?

Those are good questions, I think. There's something odd that happens with "knowing" characters: I can get characters right without any guessing on Skritter, but still not quite know them "in the wild". I guess it is because of the way the mind works: knowing is, to some extent, contextual. Which is why I can also forget people's names when they turn up in odd places...

I think that Nick said a while back that a character counts as "known" once the scheduling interval passes a certain threshold - 12 hours, I think. So if your interval between repetitions climbs over 12 hours, it will count as "known". And conversely, if it slides below 12 hours (because you've gotten it wrong a few times), it will no longer count as "known" - so your character count can actually go down (speaking from experience here :-(

Yup. I've seen those downward curves when I've been away from Skritter for a while...

Yes, that would be four items added, and the threshold is currently 12 hours.

When we get pinyin and definition practice, that would mean that there would be four possible items per character and four per word, all scheduled almost independently for maximum learning precision. You would not necessarily need to add all of those parts, though.

We'll probably even make it so that you can turn off the writing part, if you just wanted to study definitions and pinyins, for example.