Placeholder parts are 1-line dimensionless parts used for preliminary designs. When you draw systems with placeholder parts, the software inserts segments and fittings at appropriate points like other duct types. You can convert placeholder parts to any other part type, such as commercial steel or copper tubing.
To draw a system with placeholder parts, configure a routing preference using placeholder parts in the default part catalog. The default Aecb Model drawing template includes a placeholder routing preference. Draw systems just as you would for any other duct type. To convert the placeholder parts to other parts, modify the part subtype as explained in Modifying a Duct System.