NERC reliability standards govern transmission planning and TPL-001 is a critical standard for conducting studies. TPL-001 mandates grid testing under diverse conditions and various planning events (categorized as P0-P7). For any identified shortcomings, corrective action plans must be established. Understanding these categories helps developers understand the testing conditions used to determine the impact their projects have on the grid.
P0-P7 categories help clarify the testing conditions that your project must pass to ensure grid reliability. For example, P0 says "test the system under system normal conditions (N-0)", P1 says "test the system with single elements out of service (like a transmission line)" etc. Essentially the categories describe the contingencies that are taken during the analysis.