Official Description
6.7.1 An expression denotes a value unless a variable denoted by a variable-access contained by the expression is undefined at the time of its use, in which case that use is an error.
Simplified Description
In other words, it is an error to use a variable with an undefined value in an expression.
Error Handling
This error is reported only if checking for undefined values is enabled and the variable's type is one of the supported types (see Run-Time Project Options in the Irie Pascal User's Manual).