Each component has its own coordinate system. The hierarchy works as follows: 1. Solid body resides within a design 2. A design can be designated as a component, which resides within an… 3. Assembly (or subassembly if part of a larger assembly) The most important thing is to define your assemblies using constraints such as Align and Orient. This will prevent all of the unintended moving. It’s possible to move a solid body within its own frame of reference (relative to its own coordinate system). If you do this, and your assembly is not held together by constraints, you’ll see the body move around in the assembly.