It is actually in the Manual under Transmission and is specifically related to Progressive Range Selection as part of SelectShift Automatic Transmission. SelectShift Automatic transmission feature has two modes: Progressive Range Selection (PRS) and M mode.
Progressive Range Selection - PRS
Progressive Range Selection gives you the ability to lockout gears from the automatic shifting range. This may provide you with an improved driving experience, for example, in slippery conditions or when experiencing a steep slope.
With the transmission selector in drive (D), push the – button to active PRS. The available and selected gears are indicated on the instrument cluster.
All available gears display with the current gear indicated. Push the – button again to lock out gears beginning with the highest gear. Example: push the – button twice to lock out 10th and 9th gears. Only the available gears are displayed and the transmission automatically shifts between the available gears. Push the + button to unlock gears and allow the transmission to shift to higher gears. The transmission shifts automatically within the gear range you select.