In 2017 the Kodokan recognised 3 new osaekomi-waza classifications:
Ushiro-kesa-gatame is a long established variant of kesa-gatame, demonstrated in numerous Kodokan materials.
Ura-gatame has historically referred to multiple pins, but in modern use refers to a hold with one's back to uke and control of the near arm and leg.
However I have not encountered any official materials actually defining uki-gatame. Mikinosuke Kawaishi historically used the term to refer to the "knee on belly" position,1 but numerous modern judo videos 2 3 demonstrate the "S-mount" instead.
How do the Kodokan (and/or IJF) define uki-gatame?