OK, I was thinking about fully certified module that the user could use 'off the shelf', like the USB modules so got a bit confused. As far as I can see if you were selling a system using a module you would need to get the whole system certified, and in order to fast track the certification of your system you could use a 'pre-certified' radio module. You would need to use exactly the same antenna, modulation technique and timing that the original module was certified with. I also think the end user should not be able to easily modify things (hence the reverse thread antennae that are now in common use!). In reality this means using the same firmware. You still have to certify your final product, but if you're compliant with using the module in that way the process is faster and cheaper.
So it appears pre-certified modules simply show that it would pass given it is used in a very specific way and can speed things up and make the process of certification cheaper. It's risk reduction to getting full certification, the module manufacturer has already shown it can pass, and reduces the risk that firmware that isn't the same but still follows the rules would also pass.
There are rules you need to follow if designing your own module for pre-certification though, such as power regulation on board and using a metal can over the RF stuff. I found this handy resource. BTW it appears form this that the module has to be stand alone and not part of another system, I read that to mean only a buffered data interface to the outside world and not a control interface to the radio chip. That would appear to mean using an MCU on the module.
https://emcfastpass.com/rf-modules/Mark.
