All data types can have the value UNSET which indicates that a variable does not have a value. A variable created without giving it an initial value in fact has the value UNSET:

Variables with an UNSET value can be passed around and assigned, but use of an UNSET value where a valid item of data is required will always result in a PML error.

The presence of an UNSET value may be tested either with functions or methods:

An UNDEFINED variable is one that does not exist. The existence of a variable may be tested with these functions: