* fails, or is cancelled. */
libusb_transfer_cb_fn callback;
- /** User context data to pass to the callback function. */
+ /** User context data. Useful for associating specific data to a transfer
+ * that can be accessed from within the callback function.
+ *
+ * This field may be set manually or is taken as the `user_data` parameter
+ * of the following functions:
+ * - libusb_fill_bulk_transfer()
+ * - libusb_fill_bulk_stream_transfer()
+ * - libusb_fill_control_transfer()
+ * - libusb_fill_interrupt_transfer()
+ * - libusb_fill_iso_transfer() */
void *user_data;
/** Data buffer */