To understand income from investment properties first you need to look at the types of Rents and Leases.
Not all commercialrents and leases are the same. to understand income from your investment property you should know that there is the contracted rent, anticipated rent, and market rent. There is also percentage rent, overage rent, and surplus rent. Then there is the question about what expenses is the tenant responsible for and what is paid by the landlord? These questions are addressed in the type of lease.