That's not how it works. Calling a property and asking them to put U1 on your reservation will not entitle you to or boost your chances at an upgrade. In fact, depending on who you speak to at the property, awareness of where to put that code, or what you are even talking about will likely be slim to none.

These are internal codes for Marriott's PMS system. They also serve as a quick reference guide when someone at the property is looking at your reservation. Most of this information is passed along directly from MARSHA to the property specific PMS system.