What happens if a variable referenced in an Appian expression is not defined?

In Appian, if a variable referenced in an expression is not defined, it results in an error being thrown. This is important because Appian expressions rely on defined variables to operate correctly. When a variable is missing, the expression engine cannot resolve the reference, leading to a runtime error. This behavior helps developers identify issues in their expressions, ensuring that all necessary variables are properly initialized or defined before they are used in calculations or logic.

The other choices suggest outcomes that do not accurately reflect Appian's behavior in this situation. The expression executing without a defined variable would lead to incorrect results, pausing a process is not triggered by undefined variables, and the assumption that a variable would default to null does not apply since the absence of a definition generates an immediate error instead.