An Organizer will use a side letter to provide one SPV investor different (usually improved) economic terms than another investor or the rest of the SPV investors. Sometimes an SPV Organizer will reward or entice an investor to subscribe to the SPV with a change in the SPV economics, like lower carry or management fees. Because the SPV documents state the economic terms for all SPV investors, if the Organizer wants to give different economic terms to one or a few of the investors they don’t change the SPV documents, they provide a side letter that states this investor has different terms. Like a quiet side contract. See MFN.