How is the USB_HP_CAN_TX triggered

[VirtualNonsense]

I tried to adjust this example to fit my purpose, however i don't get how this event works.

stm32f1xx-hal/examples/usb_serial_rtic.rs

Line 79 in aadcd21

#[task(binds = USB_HP_CAN_TX, shared = [usb_dev, serial])]

As far as i understand it, it should trigger when there is data to send via the usb port, but im not sure how to assigne a buffer for it to watch.
Has anyone successfully used this event for this purpose or is my understanding even more limited than i assumed 😅