It means you should make commercially sound decision when resolving issues or exploiting opportunities.
To demonstrate this skill you should have knowledge of external environment in which organisation operates.
Commercial acumen is nothing but a common business sense. It depends on how good you understand a Business/ Industry/ Supply chain etc.
Even auditors need good commercial acumen.
But the part of commercial acumen is not the thing which you can find in a book you can develop that skill by reading the business magazines or being continuously updated with latest news or trends