Em comparação com a função de pesquisa de sites:
Minha especulação é que a busca no site acomoda alguém procurando por "BBJ" de maneiras diferentes (porque, eu não sei), ou o banco de dados precisa de limpeza. Como você pode ver, dois dos três são iguais. E a julgar pelo designador real, eles são todos iguais.
Quanto às regras:
Eles podem ser encontrados no ICAO DOC 8643, Designers de Tipo de Aeronave; ou FAA JO 7360.1B - Designadores de tipo de aeronave. A Ordem de Serviço da FAA tem 2 meses e também contém a lista completa.
A parte relevante para esta questão:
A different designator for an alternate or subtype version of the aircraft will only be allocated when there is a significant difference in performance for ATS or no shared designator can be assigned.
(ênfase minha)
Isso significa que o A320neo tem uma mudança de desempenho significativa (A20N vs. A320). Um avião como o Boeing 777-200 com três opções de motor não precisa de designadores diferentes. O A330neo ainda está para voar, mas o A339 aparece na pesquisa do site. Nada surgiu para o A330-800neo.
Relacionados: O que O ID 737 MAX 7, 8 e 9 ICAO será?
As regras completas:
2-2. How Designators are Formulated. The International Civil Aviation Organization (ICAO) has the role of creating standard aircraft type designators for aircraft that commonly request air traffic service. ICAO assigns an aircraft type designator using the following principles:
a. Only one designator will be assigned per aircraft type.
b. A designator will be derived from the manufacturer’s model number or name, or from a common military type.
c. The designator will not be longer than four characters and will generally begin with a letter.
d. An assigned designator will not be changed for license-built aircraft, when an aircraft model is sold to or manufactured by another company, when the manufacturer’s name changes, or when the aircraft type is derived or converted from another type.
e. A different designator for an alternate or subtype version of the aircraft will only be allocated when there is a significant difference in performance for ATS or no shared designator can be assigned.
f. “Homebuilt,” “amateur-built,” or “kit plane” aircraft that exist in operationally significant numbers will be assigned a designator; however, these aircraft will only be listed under the original designer or under the manufacturer that produces or produced the aircraft type in series.
g. In general, an aircraft type designator will be assigned to all aircraft heavier than micro-/ultra-light. For the purpose of ICAO Document 8643:
Micro-/ultra-light are those aircraft types with a maximum certified take-off weight of 1,000 pounds or less and a stall speed not greater than 35 knots.
Micro-/ultra-light helicopters and gyrocopters are those helicopters or gyrocopters with a maximum certified take-off weight of 1,000 pounds.
Micro-/ultra-light autonomous, optionally piloted, and remotely piloted aircraft are defined as having a maximum certified take-off weight of 330 pounds or less and a stall speed not greater than 35 knots.
Micro-/ultra-light autonomous, optionally piloted, and remotely piloted helicopters are defined as having a maximum certified take-off weight of 330 pounds or less.
h. Algarismos romanos usados como parte de nomes ou números de modelos de aeronaves serão substituídos por números arábicos.