How to Create Custom Calculations in FCCS with Fix& If ? Differences to Planning

Summary:

I am struggling setting up a custom calculation rule in FCCS. I have the task to add some special local gap calculations. For this the start is to copy the data in the custom1 dimension. Can somebody please explain me the secrets/ differences to planning? It appears that 5 dimensions are special, means we are not allowed to reference them in Fix? How to I reference them correctly in IF Statement using prompts or substitutionvariables?

You can find sample code below, which always generates validation errors

I followed examples from best practice guide:

Configurable calculations best practices for Financial Consolidation and Close (FCC) for Enterprise Performance Management (EPM)