USDA Loan Confusion

[Jennifer Oquendo]

How can you use a USDA to invest? 

I'm currently reading 'the book on investing in real estate with no and low money down' by Brandon Turner. And I'm reading about the USDA loan, (in the book, it states that its only for rural single family homes only, but reading through the forum, I see that you can buy a multi-family property with a USDA loan, depending if the location of the property is eligible. Is this accurate? 

If it's only for rural single family homes, how are you investing, if you must reside in the residence?