The general policy also applies.
The goal is to avoid the creation of a flat and unmanageable namespace.
If you want other types of resource records, you will have to request a domain delegation for the desired zone and manage the new zone yourself.
This implies that you need access to a machine running a name server. If you don't, you can try to find DNS hosting.
3 character names are normally reserved for internal use and future extensions, but can be allocated on a case-by-case basis preferably to non-profit associations, European organizations, and so on.
2 letter names are reserved to domains completely open to registration and future internal use. The first category includes:
Write to hostmaster@EU.org if you have good reasons to ask creation of a domain on these criteria (for example if you want to obtain and manage a delegation for a country domain).