VBATT is needed if you are using FX3 as a USB host. That means you need to connect a battery circuit to supply VBATT.
If you are using FX3 as a USB device then VBUS should be good enough to power the USB I/Os.
Even if you connect both VBUS and VBATT, FX3 will consider VBUS as the primary supply as soon as it becomes more than 3.7V.
Please let me know if this is not clear.
I'm clear. Thank you!