Circuit explanation, role of the trimpot and capacitor, as shown in a part of the circuit
I think the confusion arises because the schematics is poorly drawn.  C5 is connected between 3.3V and Ground, while the 3.3V is also connected to pin 35 -> it is a decoupling cap.  the trim pot is connected as a variable resistor from the 5V rail and VBus, so it is acting as a Pullup resistor.  In the USB spec the various pulllup's on various pins allows for setup information to be determined by the various devices - so it looks like this meant to be adjustable for either experimentation reasons or other more subtle reasons.

As an aside, the floating end of the TRIM Pot should be connected to the wiper.

Typically the D+/D- lines have pullups to indicate speed class, but there I can't find the place in the spec where the Vusb is pulled up.

USB  V2.0 spec here from the source

You can tell this is a high speed connection because of the lack of  Pullup resistor on the D+/D- lines.  My best guess is that the resistor on the Vusb is mean as a current limiter to ensure that only self powered devices attach.  If it draws any amount of power the voltage drops and the device will reset itself.  But someone with a more intimate knowledge of the spec will ave to comment.