Using the Section definition dialog, you can define or modify a section for a predefined section type:

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 Rectangular opening  | 
 H = height B = width  | 
 
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 Round opening  | 
 D = diameter  | 
 
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 L-shaped opening  | 
 h, H = heights b, B = widths  | 
 
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 T-shaped opening  | 
 h, H = heights b, B = widths  | 
 
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 Section of an arbitrary shape:      | 
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To add a new section to the list of sections available in Autodesk AutoCAD Structural Detailing - Formwork Drawings:
When defining an arbitrarily-shaped section, specify the name of the cross-section, and click either Define or Select.