Which aircraft features the identification number 6?

The A321 NEO is recognized with the identification number 6 due to its enhancements and modifications over the standard A321 model. The NEO, which stands for "New Engine Option," incorporates more efficient engines and advanced aerodynamics, leading to improved fuel efficiency and performance. This aircraft features the latest technologies and superior performance characteristics, making it distinctively identified within its family.

The A320, A321, and A320 Spaceflex, while related, do not carry this specific identification as they do not possess the significant updates that characterize the NEO version. The A321 NEO's advancements in fuel efficiency and its aerodynamic improvements are key factors in its designation, marking a clear evolution from earlier models in the A321 series. This differentiation is what makes the NEO stand out in the context of aircraft identification numbers.