What are “open 2LDs” and “closed 2LDs”?

The .au domain is sub-divided into a number of 2LDs (Second Level Domains). .AU 2LDs are divided into 2 categories, open 2LDs and closed 2LDs. Open 2LDs are those that are basically open to all users, subject to some eligibility criteria. Closed 2LDs are those with a defined community of interest that are closed to the general public. It is not possible to register domain names as 2LDs directly under .au.