This issue is to answer the open question from Wade.

You might already have read lots of document regarding CDS extensibility, let me use a real example to demonstrate.
You can follow this link to create extension field on MARA.
According to guideline we have already gone through, every extensible CDS view must define an extension include view. In S4 product related topic, it is E_PRODUCT, the view you are working in this wiki.
One example of extensible CDS view is I_PRODUCT, in transaction code SCFD_REGISTRY, you can find the usage registration on E_PRODUCT below:

After I have created extension field in MARA,

The extension field will also appear in extension view:

As a result this field will also appear in SQL view of extension include view:

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