Undocumented property "posture"?

SelfClass has a property posture which is set to 0 whenever the player exits an object, and if it's non-zero before this it affects the exit message to say "stand up" rather than "get off" or "get out of". So it seems if this is non-zero the player has adopted some non-standing posture, like sitting, lying down or perhaps kneeling. However, the library never sets this to a non-zero value and I fail to find any mention of posture in documentation or release notes.

Perhaps this should be complemented with a (modifiable) set of postures like sitting and standing, and a way for a supporter or container to communicate which postures are possible for this object and which posture is the default. Library messages could then also automatically adapt to the different postures.