Recall that callback is executed before and after the endpoint specified by type, endpoint, and direction transfers data. Below is its function prototype:

typedef void (usb_callback_t)(struct usb_xfer *, usb_error_t);

Here, struct usb_xfer * contains the transfer state:

struct usb_xfer {
...
        uint8_t         usb_state;
/* Set when callback is executed before a data transfer. */
#define USB_ST_SETUP            0
/* Set when callback is executed after a data transfer. */
#define USB_ST_TRANSFERRED      1
/* Set when a transfer error occurs. */
#define USB_ST_ERROR            2
...
};

Generally, you’d use struct usb_xfer * in a switch statement to provide a code block for each transfer state. Some example code should help clarify what I mean.