Mass loss! So what?

jinwon park

I have questions on the loss of mass. In the literature, some researchers insist that "this method causes the loss of mass while our method is mass-conservative method". In fact, I do not understand the physical meaning of the loss of mass. If mass is lost in the computation, what happened? What's wrong with the mass loss? And how can we figure out whether the scheme is mass-loss or conservative? Do you know how to measure the loss of mass?

Thanks.