NULL

This is somewhat of a curiosity item in ASN.1, because prior to single-value subtyping contains only one value. It also provides only one instance in ASN.1 where the same notation - NULL - serves to denote both a type and a value.

It may be thought that one-value type may be useless, because of carrying no information, but it is very useful under some special circumstances. For example, the folowing type

OptionalInteger ::= CHOICE {INTEGER, NULL}

can be defined when an integer value is not always present to be sent. However, there is another way for doing that in ASN.1 by OPTIONAL keyword, so NULL is slowly forgotten.