About the Assemble Step Type
Use an Assemble step to assemble parts, assemblies, and individual components from reference assemblies into a process assembly model, as follows:
Select reference components from the most recently accessed graphics window (the active window). If the object in the active window is an assembly, you must select the specific components to assemble in this step. You can also select components to assemble from a partially assembled or disassembled reference assembly.
Selected process assembly components are displayed as solids in the graphics window. If you have added components without retrieving a reference assembly, select the components to assemble from the Model Tree.
You can assemble another occurrence of the same top-level component in an assembly from the original model, or you can assemble further instances of the component from the instance that has already been assembled.
Set the display_comps_to_assemble configuration option to  no (default is yes), to enhance performance when working with complex assemblies. You can then select design assemblies and components from the Model Tree without displaying them on the screen until they are selected.
Click Simplfd Rep in the STEP: Assemble dialog box to define a simplified representation of the process assembly, as you would in Assembly mode.
You can create a process assembly to use as a Bill of Materials. Each time you place a component that is part of an assembly model, and the assembly model is itself included in the process assembly, a new instance of the component is added to the BOM.
If there are no assembled features or components in the process assembly model, or if all components have been disassembled, a new component is assembled in the default orientation and location, as would happen with a regular assembly in Assembly mode. If there are assembled features or components in the process assembly model, click Model > Component > Package to access the Component Placement dashboard and place additional components.