Flexible PVC, often referred to as "flex PVC" or "spa flex," can be used in some pool plumbing applications, but it's not typically used for the main plumbing lines of the pool. Instead, it is more commonly used for shorter runs, connections, or in situations where flexibility is required, such as connecting pool equipment like pumps and filters. Rigid PVC (Schedule 40 or 80) is generally preferred for the primary plumbing lines of the pool because of its durability, resistance to corrosion, and ability to maintain water flow efficiently over long distances. Flexible PVC can be a suitable choice for specific scenarios within the pool plumbing system, but it's essential to follow local building codes and manufacturer recommendations when selecting and installing plumbing materials for your pool.