In the video I posted earlier Jerry Yeung talked about between the shoulders.
Tan is used to spread energy and deflect attacks coming in. To me, this really does seem to be not limited, but most effective against attack where your opponents striking elbow is between your shoulders. I think there's better techniques for attacks coming from outside your center of mass. You can use tan to stop a hook, but it's just that. There's no "spreading" a decent hook. But not everyone goes strictly by the "spread" definition.
Yes, I agree. From a anatomy/biomechanics standpoint a Tan is not a strong enough structure to try and use as a force on force block....like stopping a wide swinging punch or hook. As you point out, there is no "spreading" involved when used that way. I would use a Lan instead. Lan means "obstruct." So one of Lan's uses is as a block or obstruction. And Lan does not have to always be parallel to the ground as people do it in their Chum Kiu form.