Here is a list of all documented variables with links to the struct/union documentation for each field:

- a -

actual_length : libusb_iso_packet_descriptor, libusb_transfer
altsetting : libusb_interface

- b -

bAlternateSetting : libusb_interface_descriptor
bcdDevice : libusb_device_descriptor
bcdUSB : libusb_device_descriptor
bConfigurationValue : libusb_config_descriptor
bDescriptorType : libusb_bos_descriptor, libusb_bos_dev_capability_descriptor, libusb_config_descriptor, libusb_container_id_descriptor, libusb_device_descriptor, libusb_endpoint_descriptor, libusb_interface_association_descriptor, libusb_interface_descriptor, libusb_platform_descriptor, libusb_ss_endpoint_companion_descriptor, libusb_ss_usb_device_capability_descriptor, libusb_usb_2_0_extension_descriptor
bDevCapabilityType : libusb_bos_dev_capability_descriptor, libusb_container_id_descriptor, libusb_platform_descriptor, libusb_ss_usb_device_capability_descriptor, libusb_usb_2_0_extension_descriptor
bDeviceClass : libusb_device_descriptor
bDeviceProtocol : libusb_device_descriptor
bDeviceSubClass : libusb_device_descriptor
bEndpointAddress : libusb_endpoint_descriptor
bFirstInterface : libusb_interface_association_descriptor
bFunctionalitySupport : libusb_ss_usb_device_capability_descriptor
bFunctionClass : libusb_interface_association_descriptor
bFunctionProtocol : libusb_interface_association_descriptor
bFunctionSubClass : libusb_interface_association_descriptor
bInterfaceClass : libusb_interface_descriptor
bInterfaceCount : libusb_interface_association_descriptor
bInterfaceNumber : libusb_interface_descriptor
bInterfaceProtocol : libusb_interface_descriptor
bInterfaceSubClass : libusb_interface_descriptor
bInterval : libusb_endpoint_descriptor
bLength : libusb_bos_descriptor, libusb_bos_dev_capability_descriptor, libusb_config_descriptor, libusb_container_id_descriptor, libusb_device_descriptor, libusb_endpoint_descriptor, libusb_interface_association_descriptor, libusb_interface_descriptor, libusb_platform_descriptor, libusb_ss_endpoint_companion_descriptor, libusb_ss_usb_device_capability_descriptor, libusb_usb_2_0_extension_descriptor
bmAttributes : libusb_config_descriptor, libusb_endpoint_descriptor, libusb_ss_endpoint_companion_descriptor, libusb_ss_usb_device_capability_descriptor, libusb_usb_2_0_extension_descriptor
bMaxBurst : libusb_ss_endpoint_companion_descriptor
bMaxPacketSize0 : libusb_device_descriptor
bmRequestType : libusb_control_setup
bNumConfigurations : libusb_device_descriptor
bNumDeviceCaps : libusb_bos_descriptor
bNumEndpoints : libusb_interface_descriptor
bNumInterfaces : libusb_config_descriptor
bRefresh : libusb_endpoint_descriptor
bRequest : libusb_control_setup
bReserved : libusb_container_id_descriptor, libusb_platform_descriptor
bSynchAddress : libusb_endpoint_descriptor
bU1DevExitLat : libusb_ss_usb_device_capability_descriptor
bU2DevExitLat : libusb_ss_usb_device_capability_descriptor
buffer : libusb_transfer

- c -

callback : libusb_transfer
CapabilityData : libusb_platform_descriptor
ContainerID : libusb_container_id_descriptor

- d -

describe : libusb_version
dev_capability : libusb_bos_descriptor
dev_capability_data : libusb_bos_dev_capability_descriptor
dev_handle : libusb_transfer
direction : libusb_ssplus_sublink_attribute

- e -

endpoint : libusb_interface_descriptor, libusb_transfer
events : libusb_pollfd
exponent : libusb_ssplus_sublink_attribute
extra : libusb_config_descriptor, libusb_endpoint_descriptor, libusb_interface_descriptor
extra_length : libusb_config_descriptor, libusb_endpoint_descriptor, libusb_interface_descriptor

- f -

fd : libusb_pollfd
flags : libusb_transfer

- i -

iad : libusb_interface_association_descriptor_array
iConfiguration : libusb_config_descriptor
idProduct : libusb_device_descriptor
idVendor : libusb_device_descriptor
iFunction : libusb_interface_association_descriptor
iInterface : libusb_interface_descriptor
iManufacturer : libusb_device_descriptor
interface : libusb_config_descriptor
iProduct : libusb_device_descriptor
iSerialNumber : libusb_device_descriptor
iso_packet_desc : libusb_transfer

- l -

length : libusb_interface_association_descriptor_array, libusb_iso_packet_descriptor, libusb_transfer

- m -

major : libusb_version
mantissa : libusb_ssplus_sublink_attribute
MaxPower : libusb_config_descriptor
micro : libusb_version
minor : libusb_version
minRxLaneCount : libusb_ssplus_usb_device_capability_descriptor
minTxLaneCount : libusb_ssplus_usb_device_capability_descriptor

- n -

nano : libusb_version
num_altsetting : libusb_interface
num_iso_packets : libusb_transfer
numSublinkSpeedAttributes : libusb_ssplus_usb_device_capability_descriptor
numSublinkSpeedIDs : libusb_ssplus_usb_device_capability_descriptor

- o -

option : libusb_init_option

- p -

PlatformCapabilityUUID : libusb_platform_descriptor
protocol : libusb_ssplus_sublink_attribute

- r -

rc : libusb_version

- s -

ssid : libusb_ssplus_sublink_attribute, libusb_ssplus_usb_device_capability_descriptor
status : libusb_iso_packet_descriptor, libusb_transfer
sublinkSpeedAttributes : libusb_ssplus_usb_device_capability_descriptor

- t -

timeout : libusb_transfer
type : libusb_ssplus_sublink_attribute, libusb_transfer

- u -

user_data : libusb_transfer

- v -

value : libusb_init_option

- w -

wBytesPerInterval : libusb_ss_endpoint_companion_descriptor
wIndex : libusb_control_setup
wLength : libusb_control_setup
wMaxPacketSize : libusb_endpoint_descriptor
wSpeedSupported : libusb_ss_usb_device_capability_descriptor
wTotalLength : libusb_bos_descriptor, libusb_config_descriptor
wValue : libusb_control_setup