When mass flow takes opposite to the intended direction, a warning of ‘Negative Mass Flow’ will be thrown at us by the software. In the legend, it appears as a negative number (e.g. -10 kg/s). Negative mass flow rate can be obtained in a heat exchanger when the cold fluid is placed on the secondary side etc.
There can be other reasons for other components to result in having a nagative mass flow rate.
It is a serious warning and cannot be ignored.