Hello David
As far as routers go, the definitive answer from Cisco is:
The factory-default setup for the configuration register is 0x2102. This indicates that the router must attempt to load a Cisco IOS® software image from Flash memory, and load the startup configuration with a console speed of 9600 baud.
Cisco’s answer here is not quite so clear. There may be some variation across several platforms, but for the most part the 0x2102 is the most commonly accepted default value. The following document is an excellent resource concerning the config registers for Cisco routers:
There you can see the meaning of particular values of the register, as well as of each of the 16 bits in the register field. The following is also a nifty little tool that tells you what each register value does based on the values of each bit in the sequence:
http://www.hopasaurus.com/CISCO_confreg.html
The discussion so far is for Cisco routers. Cisco switches also have a configuration register, but it plays a much less significant role. Many switch models have a hardwired value of 0xF or 0x0 and these play no role in the way the switch boots up. These are just placeholders. Other models such as the Catalyst 6500 series, do use the register for booting, but the values have different meanings and have fewer options than for routers. Conversely, on the Catalyst 9300 for example, the configuration register is not used and has no meaning. For this reason, the documentation of each switch should be consulted for the impact of the config register on that particular platform and IOS type and version.
Now as for the values we see in the CML switches and routers, I have been unable to find any documentation concerning this. However, my hunch is that because these are virtual devices, there is no significance to the actual value used. The bootup process of these devices does not involve the parameters set by the configuration register, thus I believe this can be ignored. If anyone else has more info about this, it would be great if you can share it!
I hope this has been helpful!
Laz