TPS25751: What does the GUI USB3 Rate setting do?

Part Number: TPS25751

Tool/software:

The TPS25751 GUI has a setting called "USB3 Rate" in the "Port_Config" tab.  These are bits 14:13 of the Port Configuration (0x28) register.  The TPS25751 Technical Reference Manual describes these as "reserved" (Table 3-12).
What does this setting control?
Thanks.