Those measure differently. Reach is bought. Passability has to be observed, and it has to be observed under constraint, because an environment offering twenty things to share tells you very little about any one of them.
The cleanest instrument is deliberately narrow. One media unit. One recipient. One pass. Record who passed it, when, how long they held it, and whether the chain continued. What you get is depth, breadth across distinct social clusters, and the speed at which it crosses from one group to another.
Those numbers answer a question ad spend cannot. Which asset survives more generations of transfer. Which groups consider it worth their own credibility to hand on.
The limit is worth stating plainly. Propagation is not demand. Something can travel well and sell nothing. It is a high-signal test, not a simulator, and it is cheaper to run than the campaign it informs.