The Reynolds transport theorem provides a framework to relate the time rate of change of an extensive property within a system to that in a control volume, which is crucial for analyzing fluid dynamics. Extensive properties, such as mass, velocity, acceleration, temperature, and momentum, can be expressed in terms of the mass of a fluid portion. These properties are called extensive because they depend on the system's size, while intensive properties are their corresponding values per unit mass.

The total amount of an extensive property in a system at any given moment is the sum of the amounts associated with each infinitesimal fluid particle. The time rate of change of this property is determined by differentiating it with respect to time. In a control volume, the rate of change of the property can similarly be expressed by considering both the time-dependent changes within the volume and the net flux across the control surface. Flux represents the rate at which an extensive property moves through a unit area of the control surface, providing a link between the system and control-volume perspectives.

An important aspect of the Reynolds transport theorem is recognizing that even when a control volume and a system temporarily occupy the same space, the quantities of the extensive properties within them may differ due to the continuous flux across boundaries. This distinction is fundamental in fluid mechanics and control volume analysis, especially for systems in motion.