There are some categories of hardware, such as P-ATA controllers, LPC controllers or watchdog times (in servers) tat do not fit in any available category.
Example:
lspci -nnk
00:1d.4 System peripheral [0880]: Intel Corporation 6300ESB Watchdog Timer [8086:25ab] (rev 02)
Subsystem: Intel Corporation Device [8086:345c]
Kernel driver in use: i6300ESB timer
lspci -v
00:1d.4 System peripheral: Intel Corporation 6300ESB Watchdog Timer (rev 02)
Subsystem: Intel Corporation Device 345c
Flags: medium devsel
Memory at decff800 (32-bit, non-prefetchable) [size=16]
Kernel driver in use: i6300ESB timer
lspci -vmmnn
Slot: 00:1d.4
Class: System peripheral [0880]
Vendor: Intel Corporation [8086]
Device: 6300ESB Watchdog Timer [25ab]
SVendor: Intel Corporation [8086]
SDevice: Device [345c]
Rev: 02