Ich darf hier aus der Dokumentation zitieren:
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New checking option against uninitialized variables
ArchiCAD 13 introduced a new warning in GDL script checking. It can warn you about uninitialized variables.
Since GDL is derived from the Basic programming languages, you don't have to define or initialize your variables. They will be considered to be zero when first used. In many cases this is the desired function but it hides lots of problems where you misspell the name of a variable or just forget to initialize it in one branch of a difficult 'if' clause.
For a warning-free script you will have to initialize all variables in all cases. In the vast majority of the cases it will eliminate a bug and for the rest of the cases it makes your scripts more straightforward.
Quelle