When it comes to investment properties there are loopholes which create many options. For instance, Section 1031 of the IRS tax code permits real estate investors to sell their investment properties and in return allow a trade for comparable or similar matched investments in order to defer the tax as the capital gains amass. It seems that real estate is truly the most popular transaction permitted by this code. Something called the Triple Net or NNN otherwise known, as Leased Real Estate is considered appropriate as alternate property during such a transaction. What this really entails is a Net lease where a tenant foots the bill for all or most of the properties' active expenses over and above the rent. It is important that before we discuss the particulars of Net lease that we have an understanding of other kinds of leases as each serves a different purpose.
First there is the bond lease that makes the tenant completely accountable for active expenses encompassing the property's operating costs, which include regular maintenance, repairs and substitute costs for replacing materials etc. Second there is the Triple Net or NNN lease, which incurs actual restrictions on capital expenses. The tenant must pay for property expenses including tax, insurance and maintenance, as under this kind of lease, these are the tenants responsibility. Third taking from the NNN lease is the Net Net or NN lease. This is somewhat the same as NNN lease but returns the responsibility of the physical up keep of the structure to the landlord. They must make sure major items such as the roof are in good working order. Lastly the Modified Net lease infurs that the tenant pays for everything including utilities, maintenance, repairs and insurance. They do not pay property taxes.
For the NNN situation first the situation may allow less property management issues to be a problem for the investor. This is especially true for investors dealing with multi-family units, complexes considered commercial because they want the profit and income without the hard work or heartache. They also want to postpone their tax accountabilities without having to play the role of landlord 100% of the time. Savvy investors use NNN leases because they insure income but still allow for ownership to stay in their name and portfolio while maintaining a good level of capital. Another aspect of the NNN is that it also makes the transfer of real estate to beneficiaries easier.
If you are a real estate professional then you know about the gross income multiplier, GIM or the formula that determines the value of rental real estate. It has been used for decades. 
There are many schools of thought on how to determine the value of rental property but I always use my readings as a rule of thumb. Any real estate textbook will tell you not to invest in a property with a GIM of more than 8. The formula is simple enough: divide the price by the gross annual rents to get the GIM.
With this definition in mind it seems that particular author wants me to never pay more than 8 times the annual rent for a rental property. This helps the buyer open their eyes to mow much of a rental property they can afford to buy. It makes plenty of sense really. Seems that if a property was selling for 6 times the rental income, then that was a really good deal. I mean anything higher than 8 times the rental income and that becomes a dangerous proposition. This simple method makes it easy for the average Joe to know what they are getting into.