Share rate
Share rate is shares divided by reach. A share is an endorsement made in front of the sharer's own audience — the costliest ordinary interaction and the mechanism of organic spread. Content engineered to be shown to someone ('send this to…') trades on it deliberately.
Worked example
150 shares on 10,000 reach is 1.5% — each share auditioning the content to a new audience for free.
Full context in the guides, or measure it yourself with the free tools.