Problem

I can't seem to find where the Airflow property values are defined.  If I place a supply duct then the Airflow is automatically set to Supply, return is set to Return, etc..  Where exactly do those values come from?   If I wanted to create a new airflow type and associate that with a new duct type how would I do this?

Solution

These values are based on the active Family + Part.  If you change that during placement, the Status and Air Flow properties should dynamically change to reflect those values.  It is the properties schema that controls this. From the delivered Help:

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Air Flow: 

<active part name text string before hyphen>

Sets component air flow status. Data property retrieved from the Building Part definition.
Air Flow property can be any value allowed as a Building part name unless a hyphen character is encountered. Characters after the hyphen will not be added to property.

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Status:

<active part name text string before hyphen>

Sets component status. Data property retrieved from the Building Part definition.
Status property can be any value allowed as a Building part name unless a hyphen character is encountered. Characters before the hyphen will not be added to property.

Create new Parts and follow the 'hyphen' schema using the Part name to populate the properties.  For example, a Part named "Refrigeration-New" would populate the Air Flow property with "Refrigeration" and the Status property with 'New".