Is single tank capacity a linear system?

As asked the question is not very illuminating. Capacity of tanks, single or multiple is just a volume. It is a single value. Nothing linear or non linear about it.
In a cylindrical tank with axis vertical and rectangular tanks holding liquids, the current capacity varies linearly with depth of liquid.In tanks of other geometries or cylinders with axis non vertical, this relationship is non linear.

Yeah, I searched for the keywords randomly on Google in order to find an answer, but couldn't get any. A little more insight would definitely help us understand and answer your question better. I'm bit confused on the usage of word 'linear'.