§1. A synod of bishops assembled in an ordinary general session consists of members of whom the greater part are bishops elected for each session by the conferences of bishops according to the method determined by the special law of the synod; others are designated by virtue of the same law; others are appointed directly by the Roman Pontiff; to these are added some members of clerical religious institutes elected according to the norm of the same special law.
§2. A synod of bishops gathered in an extraordinary general session to treat affairs which require a speedy solution consists of members of whom the greater part are bishops designated by the special law of the synod by reason of the office which they hold; others are appointed directly by the Roman Pontiff; to these are added some members of clerical religious institutes elected according to the norm of the same law.
§3. A synod of bishops gathered in a special session consists of members especially selected from those regions for which it was called, according to the norm of the special law which governs the synod.
Source: Code of Canon Law (Vatican.va)