CDC function header type. 

This enumeration identifies the CDC function header type.

typedef enum {
  USB_CDC_FUNCTIONAL_HEADER = 0x00,
  USB_CDC_FUNCTIONAL_CALL_MANAGEMENT = 0x01,
  USB_CDC_FUNCTIONAL_ABSTRACT_CONTROL_MANAGEMENT = 0x02,
  USB_CDC_FUNCTIONAL_DIRECT_LINE = 0x03,
  USB_CDC_FUNCTIONAL_TELEPHONE_RINGER = 0x04,
  USB_CDC_FUNCTIONAL_TELEPHONE_CALL_AND_LINE_STATE_REPORTING = 0x05,
  USB_CDC_FUNCTIONAL_UNION = 0x06,
  USB_CDC_FUNCTIONAL_COUNTRY_SELECT = 0x07,
  USB_CDC_FUNCTIONAL_TELEPHONE_OPERATIONAL_MODES = 0x08,
  USB_CDC_FUNCTIONAL_USB_TERMINAL = 0x09,
  USB_CDC_FUNCTIONAL_NETWORK_CHANNEL_TERMINAL = 0x0A,
  USB_CDC_FUNCTIONAL_PROTOCOL_UNIT = 0x0B,
  USB_CDC_FUNCTIONAL_EXTENSION_UNIT = 0x0C,
  USB_CDC_FUNCTIONAL_MULTI_CHANNEL_MANAGEMENT = 0x0D,
  USB_CDC_FUNCTIONAL_CAPI_CONTROL = 0x0E,
  USB_CDC_FUNCTIONAL_ETHERNET_NETWORKING = 0x0F,
  USB_CDC_FUNCTIONAL_ATM_NETWORKING = 0x10,
  USB_CDC_FUNCTIONAL_WIRELESS_HANDSET = 0x11,
  USB_CDC_FUNCTIONAL_MOBILE_DIRECT_LINE = 0x12,
  USB_CDC_FUNCTIONAL_MDLM_DETAIL = 0x13,
  USB_CDC_FUNCTIONAL_DEVICE_MANAGEMENT = 0x14,
  USB_CDC_FUNCTIONAL_OBEX = 0x15,
  USB_CDC_FUNCTIONAL_COMMAND_SET = 0x16,
  USB_CDC_FUNCTIONAL_COMMAND_SET_DETAIL = 0x17,
  USB_CDC_FUNCTIONAL_TELEPHONE_CONTROL = 0x18,
  USB_CDC_FUNCTIONAL_OBEX_SERVICE_IDENTIFY = 0x19
} USB_CDC_FUNCTIONAL_DESCRIPTOR;

This value goes into the bDescriptorSubtype of CDC functional descriptor.