Picture a piping system made up of threaded steel pipes. It is to hold regular water at 7-10 bars. When testing the system for leaks, air is used due to a frost issue and the consequences of a larger leak with water. But what holds water doesn't hold air, and so it slowly leaks out. The question is how fast the air should be allowed to leak out before being considered "good enough"? I suppose that is dependent on the number of joints and perhaps what air pressure is applied? Usually we never apply more than 1,5 bar of air pressure during such a test.