Those fields are used as full-screen display mode preferences. By default, DXGI swap chains hook Alt-Enter to provide applications with the ability to transition from Windowed to Fullscreen. When fullscreen, those fields indicate what type of preferences the application would like. As they are only preferences, numerous other factors affect which display mode properties are actually used, like which display modes are actually available.

In particular, I believe these fields only provide information to affect the behavior of Alt-Enter entry into fullscreen, and other SetFullscreenState usage. 

IIRC, an application can opt-out of the Alt-Enter behavior, and manually override this information (i.e. be more flexible about its desires) by using ResizeTarget. I don't think these preferences can modified, but they can only be ignored through manual ResizeTarget usage. Applications can present the user with results from GetDisplayModeList to manage such preferences.