The SOC is the System Operating Code. This code is generally only actively found, or changeable, in TDMA and CDMA phones, and actively tells the phone which network it can safely operate on primarily, apart from the IRDB information.
Each carrier has it's own unique SOC. I was going to include examples, but then realized that might be seen as releasing proprietery information, which would be a fireable offense, so never mind.
Sometimes, if a customer wants to use an older TDMA/CDMA phone on a new network (because they want the service with the other carrier, but they don't want to enter into a 2 year contract or bother with the hassles of buying and learning about a new phone) a dealer will ever so helpfully and cleverly reprogram the SOC code in the phone.
The general result of this is that the phone will work fine until it hits the old network, at which point the phone will recognized the home that was programmed into it at the factory, the soc will revert, and the phone will stop working.