Route Specifications
Open at Options > Project Preferences > Route Specifications or from the MEP Toolbar.

Use Route Specifications to define
- Reference sets (e.g. DN values) for ducts and pipes
- Tables of dimensions (presets) for each MEP element type (segment, elbow, branch, transition), based on the respective Reference Set
Once you have defined dimension presets based on reference values, it is easy to create MEP routes whose elements always correspond to the sizes you need.