I have just one key fob and emergency key for my X5 and I just had my first experience with locking the key in the car. I am trying to learn more about the Comfort Access system and the ins/outs of replacement hard keys and key fobs.

I plan on buying some hard key blanks and getting copies made of the one that I have in my possession. I am looking into what my options are for getting additional key fobs.

There are all sorts of options for buying a key fob with hard key and no circuit board...I can do that. I get it.

There are all sorts of options for buying a key fob with the hard key and a comfort access capable transmitter inside...I don't know much about this yet so I'm digging.

There are reports on the forums that folks have 315mhz and 868mhz key fob transmitters. I'm guessing the difference has to do with those with keyless entry and those with comfort access, but I don't have any evidence to suggest that this is true.

Trolling fleabay it is clear that sellers offer key fobs that are one frequency or the other but I'm not sure why.

When I decode my VIN I find the following option code:
S876A - Radio frequency 315 MHz

The back of my key fob is heavily worn and I can't clearly read the printed info...but I think I see 315 Mhz

The E70 comfort access training docs say otherwise:

ID Transmitter
The ID transmitter for Comfort Access must be actuated by means of a radio signal. The ID transmitter is therefore equipped with a receiver for the coded 125 kHz radio signal that is transmitted by the outside door handle electronics module. The radio signal enables the ID transmitter to register with the vehicle (authentication).
For this purpose, the ID transmitter emits a coded 868 MHz high-frequency signal to enable identification of the ID transmitter as being valid and belonging to the vehicle.
The ID transmitter is exclusively responsible for use of Comfort Access.

The same document has no reference to 315 Mhz transmitters. My X5 is from the final month of E70 production and I'm wondering if maybe BMW was retooling from 868 to 315 as part of the F-series of X5s that rolled out later and maybe the transition happened well ahead of the F-series introduction, perhaps as part of the LCI Facelift in 2012?

Anyone have an input on why the transmitter frequency differences? Anyone have experience coding a comfort access key fob to a car without the dealer?