The ASN.1 NULL type is used when you need a placeholder for which there is no value. For example, it can be used to mark a currently empty space. The NULL type has only one possible value, also called NULL.
PlaceHolder ::= NULL currentlyUnknown NULL ::= NULL
currentlyUnknown ::= NULL
In BER, the NULL type value defined in the example above is encoded as follows:
The NULL type can be constrained by a single value.