Vietnamese authorities have proposed an interesting idea: a book about what you can sing. Right. This is going to take some explanation. But in a nutshell, it's a book of permitted songs. Until now, singers in Vietnam were unclear about what they could and could not sing for songs existing before 1975. With the publishing of the permitted songs book, for any song from 1975 to pre-history, you are now in the know.

If it still does not makes sense, an article on VietnamNet explains:

Some say the agency should issue a list of banned songs instead of licensed songs, because the number of banned songs is often less than licensed ones. However, Cuong said the list of those banned may not have songs written before 1975, which are not made public yet.

Did anyone say Sing Sing?

Source:
VietnamNet. Agency to issue list of permitted songs