A sit position is obtained when the angle of the skating leg in the knee is not more than 90°; if this angle is more than 90°, the position is considered as an upright.
If the buttocks are higher than the knee, there must be a GOE Reduction from -1 to -3 (depending on the height and the duration of such position).
(quoted from ISU communication 1396, available at the ISU Website)
I recall this was explained to us at the post-competition dinner of the last ISU Adult Competition, unfortunately, after many adults had already been disadvantaged. While a 90°-at-the-knee requirement does sound reasonable, this poses a problem for many adults who really cannot sit that low.
So now, I've to work on getting a lower backsit. Years ago, my coach had been trying to get me to wrap my free leg a little (apparently my free leg is way way way too straight!). Recently, a post on the mailing list suggested to not turn out the free hip. Perhaps I ought to try that...
Wonder if there is a GOE Increment if your buttocks are lower then the knee!