There are four types of AS_PATH segments held inside the AS_PATH PA:
The most recently added ASN to the AS Sequence is considered the process of AS Prepending.
AS_SEQUENCE is the most common type of AS_PATH PA, it contains the list of ASNs starting with the most recent ASN being added read from left to right.
The other three AS_PATH types are used for Confederations
AS_SET- this type of AS_PATH attribute summarizes routes using the aggregate-address command, allowing AS_PATHs to be summarized in the update as well.
AS_CONFED_SEQ gives the list of ASNs in the path starting with the most recent ASN to be added reading left to right
AS_CONFED_SET will allow summarization of multiple AS PATHs to be sent in BGP Updates.