RFC 1771 defines a path attribute type of AS_PATH as follows:
AS_PATH (Type Code 2): AS_PATH is a well-known mandatory attribute that is composed of a sequence of AS path segments. Each AS path segment is represented by a triple <path segment type, path segment length, path segment value>. The path segment type is a 1-octet long field with the following values defined: Value Segment Type 1 AS_SET: unordered set of ASs a route in the UPDATE message has traversed 2 AS_SEQUENCE: ordered set of ASs a route in the UPDATE message has traversed
Using a AS_PATH type of AS_SEQUENCE makes perfect sense to me: You end up with a reverse list of the ASN's you cross to get to a particular prefix.
However, I have no idea what purpose having an unordered list of ASN's between you and the prefix that would occur if you chose to use an AS_PATH of AS_SET.
The quote above is from the UPDATE message format, so presumably, an Admin has the option to send to BGP peers an ordered list or an unordered list. My question then is, what would the purpose be of sending an unordered list? In what cases would you be better off sending an unordered list as opposed to an ordered list?