An ancillary meeting is a meeting which can be in the form of a class, discussion, lecture, workshop, demonstration or similar presentation. As the name implies, ancillary meetings are subordinate to the official United Nations meetings. They are open to all delegates and the material presented becomes part of the public domain.
Professional meetings are usually called by the head of a professional or regional association and can be limited to invited guests and members. In some cases a professional meeting may be organized during the Congress in order to allow individuals of the same profession (e.g. heads of airport tower security) to meet.
Specialized meetings can be highly technical and if they cover a specific item where security is a concern (e.g. training of police cybercrime technicians) they are open only to those specifically invited.