If you have a multirail, say 4 +12 V rails rated @ 20 Amps each With say a 60 Amp max for all for rails.
You would have to insure that you did not try to connect so that a given rail did not exceed 20 Amps. Also say you had 18 Amps being used on the first 3 rails, then you would only have 6 amps for rail 4, NOT 20 Amps.
Example for +12 V Rails:
Rail 1 would go to MB. This would be for your 20/24 pin MB connector, the 4/8 Pin Connector (CPU), and +12 V For GPU (via the PCI-e connector.
Rail 2 would be to the 6/8 Pin GPU Connector.
Rail 3 would possibly be for 2nd GPU 6/8 Pin connector ( This may be from rail 2)
Rail 4 would be to all external Components (to MB), ie SSD/HDDs and DVD drives
It's possible rail 2 could be for the 4/8 pin CPU pwr, then Rail 3 would be for upto two 6/8 pin GPU pwr connectors