In siemens nx, Unite command use to combine the volume of two or more solid bodies into a single body. The target body and tool bodies must overlap or share faces.
Where do i find it?
Form menu: Insert -> combine -> Unite.
From toolbar: Feature: Unite.
How to use unite command.
1. From toolbar-> Feature -> Unite or From menu -> Insert -> Combine -> unite.
2. Select a target solid body.Target body to unite with one or more tool solid bodies.
3. Select one or more tool solid bodies to modify the target body. Tool bodies unite with target body.
4. Click Ok or apply to finish.
You an see this video below for more detail.