Definition
Source diversity is how many distinct domains, and how many kinds of domain, an answer engine draws on for a category. It is measured by counting distinct questions a domain was cited for, not by raw citation volume.
Why it matters
It tells you whether a category is winnable through your own site or only through third parties. Where citations concentrate in communities and directories, publishing more pages on your own domain will not move the answer.
How to measure it
Group cited domains by type and report the share of each type. Compare that profile per category rather than assuming one profile fits your whole market.