Propagation depth is the deepest metric of the connection function, and a direct corrective to measuring spread by reach. The usual question is how many people did this reach? The better one is:
How many generations of new capability did it create?
Teacher → student → new teacher → new students. Researcher → method → new lab → new discovery. Founder → team → new founders → new projects. A thing with vast reach but zero propagation depth (a broadcast to a passive audience) has distributed product, not capacity; a thing with modest reach but many generations of depth has genuinely multiplied what the ecology can do. Depth is what distinguishes capacity propagation from mere audience size, and it pairs with the concentration-to-propagation ratio — depth measures how far new capability travels down the generations, the ratio measures whether benefit stayed at the center or spread outward. See Measuring Spread and the Distribution Question.