Without the XTS Module, reservoirs are infinite. With the XTS module, reservoirs can be infinite or finite.

• Infinite reservoirs - infinite reservoirs refer to a massive body of fluid whose surface level does not change appreciably as a result of liquid inflow or outflow during the time frame of the simulation. An example is a large lake or the ocean.

• Finite reservoirs - (XTS Module Only) finite reservoirs (also called finite tanks) refer to a body of fluid which is small enough that its surface level changes significantly during the time frame of the simulation as a result of liquid inflow or outflow. An example is a tank which drains as the simulation progresses.

• Finite reservoirs can be open or closed. Finite open reservoirs are open to the atmosphere or some other fixed surface pressure. Typically open reservoirs have a constant surface pressure with time, although users have the latitude to pre-specify time varying surface pressure.

• Finite closed reservoirs are closed tanks in which, as the liquid level changes, the gas above the liquid expands or contracts and hence its pressure changes according to the gas thermodynamic laws. For closed reservoirs the user specifies the initial liquid level and gas pressure, but during the simulation all gas pressure and liquid levels are calculated by AFT Fathom.