Key process areas identify the issues that must be addressed to achieve a maturity level. “Each key process area identifies a cluster of related activities that when performed collectively- achieve a set of goals and enhance the process capability”( (The Capability Maturity Model: Guidelines for Improving the Software Process. Addison-Wesley. p. 32.). The KPA areas have been defined to reside at a single maturity level.
The key process areas are the requirements for achieving a maturity level. To achieve a maturity level, the key process areas for that level and the lower levels must be satisfied (or not applicable, such as software subcontract Management when there are no subcontractors).
The specific practices to execute in each key process will evolve as the organization achieves higher levels of process maturity. For instance, many of the projects estimating capabilities described in the software project planning key process area at Level 2 must evolve to handle the additional project data available at Level 3.