A fixed relation sets a 3D object's position in space so it won't move. You must select the Update Relations option in a positioning or other operation for this restriction to be in effect. However, if you move an assembly that the part belongs to, the part will move with the assembly.
For example, the red cube has a fixed relation, and the blue cube has a distance relation with the red cube. If you try to reposition the red cube with the Update Relations option active, it will not move. If you try to move the blue cube in the direction of the distance relation, as shown below, it will not move. If the red cube was not fixed, the red cube would move when you reposition the blue cube. You can move the assembly that owns both cubes, and they will both move.
To create a fixed relation,
1. Click Relations and then, in the Relations Mode, click Parametrics.
2. Click Relations and then, in the Create & Modify group, click Fixed. The Fixed dialog box opens.
3. Select the 3D Elements you want fixed. To select multiple elements, press F2 and use the Select dialog.
Parametrics.
Fixed. The Fixed dialog box opens.
to complete the operation.