I'll try.
In a fully-rated system, every protective device is rated to at least the available fault current at the service point. The advantage is that you don't have to do any homework, and you can mix brands in a system. The disadvantage is that everything costs more.
In a series-rated system, each device need only be rated for the available fault current at its terminals. The advantage is that the equipment costs are lower. The disadvantage is that you have to calculate the available fault current at each device's location in the system.
You are required to use combinations of equipment that have are certified to have been tested together and found to operate safely without damage to the equipment, and more importantly, to people and buildings.
For example, starting with the utility transformer secondary terminals, and usually assuming what's called an "infinite bus" (unlimited current) available to the primary, you use the transformers capacity, impedance, etc., and determine its available current.
Then you factor in the impedance of the service conductors from the transformer to the main disconnect, which can be rated at the available fault current at that point. You then use the impedance of the feeder conductors to rate the next equipment, etc.
This continues to the final branch-circuit breakers. You can see how this can lead to quite a savings in a larger installation. There are calculator programs that can do this easily, but you must get the transformer information from your power-company field engineer.
