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Symbol or layout representations can be found in different fields, among others in the BIM field. (Also see Section 5.12.12, “Classify Electrical Parts ”.)
Both layout and symbol representation describe abstractions of the physical 3D model. They differ both in regard to the level of abstraction and in regard to the focus conceptually. When layouts focus on the geometrical 2D representation of the physical model, symbol representations focus on the logical functionality of the model. Both concepts are not interchangeable.
Layout: According to ISO 5455:1979 layouts have the characteristic of a scale in particular, meaning the scaling factor between 2D representation and 3D model. This particularly means that for layouts sizes such as height and width can be defined, which are directly related to the 3D model.
Symbol representation: The symbol representation as logical description of a part's functionally is a deeper abstraction of the physical model, where all properties like height, width and depth, if referring to the 3D model, are being omitted. Only the circuit symbol is represented.
We can clarify the difference with the help of two transformers: The one feeds a power line, the other serves to charge the mobile phone. The layout display is very different, if the scaling factor is unchanged, which mirrors the different physical dimensions of the two transformers. In both cases the symbol representation could be the same. In the CNS classification, layouts are described by the class CNSCAX|GFX|LAYOUT and (logical) symbol representations by the class CNSCAX|GFX|SYMREP.