It could mean either, depending how you qualify it.
It basically means what the plant is capable of producing.
A plant might have 4 manufacturing units each required to produce 1 Tonne per day, so its rated capacity is 4 Tonnes per day.
If it needs one person to run each manufacturing unit, but 1 of them is off sick, then it's actual (current) capacity would be limited to 3 Tonnes per day.
Although the units may be designed to produce 1 Tonne per day, every day, a plant doesn't normally function to well if continuously operated at its maximum capacity, so they may well be designed for 1.1 Tonnes per day maximum, but still have a 1 Tonne per day rated capacity
It's similar to the difference between a car/plane's maximum speed and its cruising speed.
Hope this helps
Student or Learner