Difference between personal and commercial insurance policy for a landlord?

[Jeff L.]

What's the difference between a personal policy and a commercial policy for a landlord?

I was getting a quote for umbrella from RLI and they won't cover properties that are under a commercial policy:

• Excludes residential properties that are covered under a commercial or other non-personal premises liability policy.

But, they still cover "rentals". So what's the deal? Can you even get a rental put on a personal policy? If so what's the difference?