Cgminer will automatically find all of your BFL ASIC, BitForce FPGAs,
ModMiner FPGAs or Ztex FPGAs
The --usb option can restrict how many BFL ASIC, BitForce FPGAs or
ModMiner FPGAs it finds:

  --usb 1:2,1:3,1:4,1:*
or
  --usb BAS:1,BFL:1,MMQ:0
or
  --usb :10

You can only use one of the above 3

The first version
  --usb 1:2,1:3,1:4,1:*
allows you to select which devices to mine on with a list of USB
 bus_number:device_address
All other USB devices will be ignored
Hotplug will also only look at the devices matching the list specified and
find nothing new if they are all in use
You can specify just the USB bus_number to find all devices like 1:*
which means any devices on USB bus_number 1
This is useful if you unplug a device then plug it back in the same port,
it usually reappears with the same bus_number but a different device_address

You can see the list of USB devices on linux with 'sudo lsusb'
Cgminer will list the USB devices with the '--usb-dump 0' option
The '--usb-dump N' option with a value of N greater than 0 will dump a lot
of details about each USB device

The second version
  --usb BAS:1,BFL:1,MMQ:0
allows you to specify how many devices to choose based on each device
driver cgminer has - there are currently 3 USB drivers: BAS, BFL & MMQ
N.B. you can only specify which device driver to limit, not the type of
each device, e.g. with BAS:n you can limit how many BFL ASIC devices will
be checked, but you cannot limit the number of each type of BFL ASIC
Also note that the MMQ count is the number of MMQ backplanes you have
not the number of MMQ FPGAs

The third version
  --usb :10
means only use a maximum of 10 devices of any supported USB devices
Once cgminer has 10 devices it will not configure any more and hotplug will
not scan for any more
If one of the 10 devices stops working, hotplug - if enabled, as is default
- will scan normally again until it has 10 devices