Dear Eric :
This is an indicator of how close the computations have converged to an correct answer. If there is a lot of problem in convergence, then the conservation of mass value will indicate that. If the number is low, then the computations converged well.
If you have any additional questions or need any further information, please contact me again.
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com