From -what- BGP definitions? This one? https://www.scribd.com/document/202454953/Computer-Networking-Definitions

Lots of things get claimed in books and CS courses that are neither reflected in the standards nor match universal practice. Heck, most networking courses still teach class A, B and C... definitions which were explicitly invalidated a quarter of a century ago.

Even where authors are knowledgeable, they're constrained to present approximate explanations lest the common use get lost in the minutiae. When you want to act on the knowledge in an unusual way, you do not have that luxury. The experts in the IRTF Routing Research Group spent something like 6 years trying to find a way to filter the BGP RIB in the middle without damaging the Internet. They came up with zip. A big zero. They all but proved that it's impossible to build a routing protocol that aggregates anything anywhere but at the edges while still obeying the most basic policy constraints like not stealing transit. Forget getting BGP to do it, they couldn't come up with an entirely new protocol that did better.

Regards,

Bill Herrin