Easiest way is using “add part to assembly” or sub-assembly. But this sometime create an issue, especially if you want to export out this data to other software.
Solution: use “added material” instead.
1.With “Select Reinforcing Bar” Mode & Transparent “Ctrl+2” mode; select all rebar. Right click & click on “Detach from Part”. Repeat once again to ensure all rebar are detach from concrete panel.
2. Under “EDIT” tab, click on “Added Material”, then click “Attach Material to Part”. With Solid “Ctrl+4” mode, click on the main part, then click on the secondary part, and finish by clicking middle button (scroll) of the mouse. Repeat to add more parts to main part.
3. Again back to “Select Reinforcing Bar” Mode & Transparent “Ctrl+2” mode, select all rebar, right click and select “Attach to Part”, then click on the concrete part. Repeat to ensure all rebar are attached again.
4. Set top form face. Select the most end-side so it will capture the overall assembly/part. Notice the part name (Cast Unit Position) is now only one name.
5. Perform numbering. Re-clone drawing as needed.
The end result:
The disadvantage of using added material is that you could not edit each part individually anymore, thus this perhaps should be done only after the panel shape is finalize and confirm.