Hi,
I am not sure what the IFC value measures (I suppose the actual area of the 3D body on one side). Take the 3D model, start measuring it, and see which measurement might match the IFC value.
For the ARCHICAD value though,
here's a complete list of all the wall area parameters you can list, with pictures. The General Surface Area parameter should measure the area of 'reference line'*'height' in case of walls.
Regards,
Daniel Alexander Kovacs
Professional Services Consultant
GRAPHISOFT
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