Process Performance (Pp, Ppk) helps determine if your new process is acceptable by measuring ratios of distribution to spread and centering.
Cp and Cpk are considered short-term potential process capability measures for a process. We want to describe this quality in terms of sigma.
Process Capability & Performance (Pp, Ppk, Cp, Cpk) are indicators of the specification relative to process dispersion and process centering.
Specification limits are set by the customers; often, Voice of the Customer is referred to as the customer specification limits.
How to determine baseline project sigma is one of the key aspects of the Measure phase. It is part of the data collection plan.