Whatever the character needs it to without conscious thought. Maybe I worded that poorly, but the general concept is that it defaults to the planet (whether or not your game's planet is in motion in space. Or plane, if on a different plane.) I tend to think of the worlds in my games as moving through space, but the characters think of the ground as solid and stable, like most of us think of our planet in day to day life. If the character is on an air-ship or a sea going ship for an extended period of time and it becomes second nature for them to think of it as the most stable identifying marker around, and they need it to do some minor task, like a hand hold or a foot up I wouldn't have an issue with that. It's not breaking anything. If the character starts overthinking it, either looking for an exploit (where they acknowledge to themselves that the object it's in relation to isn't the most stable thing around) or like wile-e-coyote looking down after standing in open air for a while, I might declare that their perception of stability has shifted and as such the interpretation of immobility has. It doesn't seem more or less arbitrary to me then any other base line assumptions of immobility, and it lets them use the item a bit more. It has holes, of course, like any interpretation.