The process capability index is the amount or measure that takes place to determine how much a product can be made or capable of being made. There are certain limits, and these limits are included in the process so that the data will reveal what statistical measures are needed for this to occur.
Most of the time, CP and CPK are used together to describe this process of making this product. The main difference is that CPK takes place first, and then CP takes place next. However, they are usually kept together. CPK is when the process is not centered and CP is when the process is focused when the stable process is under statistical control. These are the minute differences between the two.