Working this out in advance with the buddy is a good idea and having a frequent buddy situation where the two of you can practically read each others minds is ideal. But neither is always possible. For example I have picked up lost buddies underwater that I never met or talked to before.
The context of the situation is important. A thumbs up at a 15 ft safety stop leaves alot of room for interpretation and there is virtually always time to discuss and clarify.
The same thumbs up at 130 ft however should normally be taken to mean something different and a little more pressing and the signal should be returned and an immediate ascent initiated since you are using air 5 times faster and will be attempting to clarify with one or more of the parties potentially significantly affected by narcosis. You can sot out specifics at a shallower depth.
And, like on the surface, approximately 80% of communication is nonverbal. If the diver giving the thumbs up is showing any signs of stress or anxiety, your response is going to be different than if the the diver is exuding coolness, unless of course you are deep and the diver is exuding coolness in circumstances where he/she shouldn't.
As for blowing off a safety stop, I would want a pretty good reason depending on the circumstances. If the buddy were down to a couple hundred pounds when we reached the stop and we were at an NDL, I'd be more likely to communicate a willingness to share air at the stop (good practice anyway) or if the offer were declined (buddy is cold etc), to communicate that I will stay at the stop and keep an eye on him as he/she surfaces and makes it to the ladder than I would be to just ascend with my thousand or so pounds of air remaining.
Overall, with a few notable exceptions and contexts, I am not a big fan of a signal automatically meaning one thing and one thing only and commanding a rigid and automatic response. Doing so, in and of itself, becomes a barrier to effective communication.
