Setup depends on who you are and what you have. If you own one rental property, you can make your tenants a customers.  If you own several properties, you can use classes to separate the properties and run separate profit and loss reports for each property.  Tenants can still be your customers.  You would classify each transaction.

If you are a property management firm, you would use the sub-customers.  If you don't have the class feature, you can use sub accts.   But it tends to make the chart of accts very large and cumbersome.