Union is one of the three boolean movements in Shapr3D. You can access it in Tools > Union. Use it to unite bodies, groups. Uniting bodies is useful for selection purposes and in the case of certain tools (e.g. Shell)
The Union flow
1. Tap on Tools > Union
2.Select the bodies, groups you want to unite.
3. Tap on Done
TIP: In the case above, the Union was necessary as we wanted to first Shell and then Fillet our body. Neither of those operations would've been the same without a union. For final design, please see picture below