One great advance in the resource-based view of strategy was the recognition by Professor Ikujiro Nonaka that some organizational knowledge and capabilities are tacit rather than explicit. As an example, think about the knowledge associated with riding a bicycle. If you wanted to teach someone to ride a bicycle, would you talk about angular momentum, center of balance, and rotational inertia? Those are the explicit physical principles that determine whether the bicycle falls down.
My guess is that's not how you learned to ride a bike. And if you have taught someone, perhaps your own child, how to ride a bike, you probably did not provide a detailed physics lesson!
Instead, you probably told the person to get on the bicycle, push the pedals, and learn how to stay up. The activity of successfully riding a bicycle, quite difficult to put in words, is a great example of tacit knowledge.
Even though the underlying knowledge and capability are tacit, the activity itself is assessable. How long did the child ride before falling down? How fast can the person cycle? How much energy is the cyclist using? As you develop Key Activities, make sure they can be measured.