A reservoir tank is a container used to store a large amount of liquid or gas, usually for later use or for maintaining a steady supply. It acts like a storage tank that can hold fluids or substances until they are needed. Full definition
Perhaps the crack on the fluid reservoir tank is superficial and you need only a new reservoir. (mechanics.stackexchange.com)
You said that there is a pin sized hole in the radiator reservoir tank, seal it up using some bondo or some other sealant. (mechanics.stackexchange.com)
Water is supplied to the target - making machine through reservoir tanks or a permanent system of pipes. (si.com)