这个packing和unpacking指的是什么意思啊

叶子62

If, for transient simulations, with steady Flow and Pressure Sources, the mass inflow rate to the system does not match the mass outflow from the flow system, and there is no evidence of 'packing' or 'unpacking' in the flow lines;
这是flowmaster帮助文档里的一句话，我实在不理解最后一句的packing和unpacking具体指的是什么意思啊，求大神解释。