Here is what I know about how it works:

You put the key in the ignition, press either LOCK or UNLOCK twice, then turn the key to ON.

The SKREEM 'hears' the signal generated by the FOB, decodes the data stream, realizes it is not one of it's prestored codes, remembers the code but does not take any action.
You then turn the key to "ON" and the SKREEM 'knows' that the code sent is intended to be stored into memory as a valid code, so it stores it, if the format is compatible with the SKREEM.

Here is what I DO NOT know:

When you turn the key to "ON", the SKREEM reads the RFID in the chip in the key (this is the coded signal that tells the SKREEM to enable engine starting). My question is this: In order to store a lock/unlock code, does the SKREEM want to see a valid RFID start code? From what I understand, it does NOT insist on a valid start code, as others have reported they have programmed keys to unlock doors that would not start the engine. This means that the only requirement is to have a FOB that sends data in the compatible format of the Crossfire SKREEM for door locking/unlocking. Apparently, that format is not all that unique to the Crossfire.