Conservation laws, conservation of mass, mass of water and air

Conservation laws and related physical laws
Conservation of mass
Conservation of energy
Conservation of angular momentum
Increase of entropy
Meaning of "conservation of X"
In an isolated system, total quantity of X do not change with time.
dX/dt = 0
In an open system, change of X is equal to net inflow of X.
dX/dt = (inflow of X) - (outflow of X)
In a steady state,
0 = (inflow of X) - (outflow of X)
What can we know from budget analyses based on conservation laws?
Estimation of the unknown term (if one term is unknown)
Estimation of accuracy (if all terms are observed)
Equivalence of mass and energy (Einstein's theory of relativity)
In our context, mass and energy can be discussed separetely.