Polygons > Merge Polygon Objects creates a single polygon object from two or more active polygon objects. Components remain separate in the merged object, but if components share a boundary edge, they can be stitched into one.
The controls are:
Name field (default Merged Polygons N) - specifies the name of the merged object that will appear in the Model Manager.
Stitch checkbox - (applicable if the component objects share a boundary edge) specifies whether to combine boundary-sharing components into one.
To merge two polygons:
Select two or more polygon objects in the Model Manager.
Click Polygons > Merge Polygon Objects.
Assign a Name to the new object.
Check Stitch if you know that the vertices on both models are exactly the same and you want them to be merged.
Click OK save the result to the Model Manager, or Cancel to exit the dialog without saving.