If it is a matter of simply turning ON and OFF a piece of geometry, then you should be able to associate a parameter to its "Visible" property. The associated parameter can have a conditional statement such as =Length > 10. If the Length parameter exceeds 10, then the Visible parameter would be turned ON for the associated geometry
1. Create the geometry to display ON/OFF.
2. Select it, and then click Properties.
3. Click the small empty rectangle (Associate Family Parameter) at the far right of the Visibility row.
4. Add Parameter - Give it a name such as the part name of the geometry.
5. Click OK several times.
6. Go to Family Types.
7. Assign formula to new parameter such as =Length > 10.
8. Create additional family types as needed.
Hope this helps.
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Mark Mates
www.prosoftnet.com