The Powder Reactivity Coefficient (PRC) value is a measurement of the interaction between the flux and solder powder. When the PRC is well outside of normal bounds, then it is more likely for printing and reflow defects to occur.
When the PRC is above the upper red limit, then there is less interaction between the flux and solder powder than what is usual for that solder paste. This could occur because the solder paste was not stirred or sheared down enough, the solder paste was expired, or the solder paste was incorrectly stored. This may cause poor rheologic behavior resulting in slump or skips and/or issues in reflow.
When the PRC is below the lower red limit, then there is more interaction between the flux and solder powder than what is usual for that solder paste. This could occur when the solder paste is opened cold or measured in high humidity and may cause electrochemical migration issues or solder balling.
Note: what constitutes a normal / usual measurement is different for each solder paste and for different applications and there is no singular "correct" control limit. It is highly recommended to tailor the control limits to your specific process. Please contact Insituware for assistance.