Confederations are groups of autonomous systems that from the perspective of other ASes appear to be a unique AS.
By dividing the autonomous system into multiple smaller ASes (SubAS) and adding new attributes to the prefixes, loops can be detected. This allows the advertisement of prefixes between routers that originally wouldn't send this information (iBGP sessions)
The prefixes within the confederation use the following additional attributes:
AS Confederation Sequence: Ordered list of the ASes within the confederation for which the route has been announced
AS Confederation Set: List of the ASes within the confederation when summarization is in place (one occurrence per AS, no specific order). Similar to the AS Set regular attribute.
The next-hop and local preference attributes are preserved in the SubAS communications (cBGP).
Within SubAS (cBGP), it's checked that the neighbor is directly connected.
Usually, Private AS numbers are used (64512-65355)