How do I choose or change an American Airlines seat without confusing Preferred and Main Cabin Extra?

What this answer depends on

• American Airlines and applicable American Eagle seat products; no promise for partner-operated flights or every fare
• Preferred is not another name for Main Cabin Extra. Decide whether your goal is a location within Main Cabin or the extra-legroom product before approving a charge. Neither label, by itself, changes a Main Cabin ticket into a premium cabin. Read the actual selection and its price rather than inferring benefits from a seat-map color.
• American says a seat can be selected later or assigned at check-in when it was not chosen during booking. That does not promise a particular seat will remain. Keep the ticket and seat tasks separate, and use the actual reservation’s available choices instead of concluding that the ticket is invalid solely because no seat was chosen.
• American’s Preferred-seat conditions apply to the specific flight. A favorable seat on one leg does not establish the same product, charge or benefit on the next. Preserve the confirmation for the flight on which the seat was purchased.
• American distinguishes regular Main Cabin seats from Preferred seats by location within Main Cabin.
• Main Cabin Extra provides extra legroom within Main Cabin rather than a separate cabin.
• American allows later selection or check-in assignment when no seat was chosen and does not guarantee an exact seat against operational changes.
• American Preferred-seat purchases apply to the selected flight and are not transferable as an entitlement to another flight.