You can select objects for Combine in the structure tree. You can also box select
multiple solids or surfaces to merge them in one operation.
Solids can be merged with solids, and
surfaces with surfaces. Solids and surfaces can only be merged if the
surfaces make a region that can be added to or cut out of the
solid.
Click the Select Bodies to Merge tool guide or hold the
Ctrl key.
Select the solid(s) or surface(s) that you want merged with the
target.
Tip:
Hold Ctrl and select the solids or surfaces you want to
combine, and then click the Combine tool to merge
them.