This section explains how to constrain a part within an assembly using Smart Dimension so that it follows another part.
Procedure:
[1] Create dimension between the assembly and the other part.
[2] Right-click on the dimension and enable
[Lock] from the menu.

[3] Select the SmartDimension, then click the small downward triangle icon.

[4] Select the part within the assembly as the driven object.
Please refer to the video below.
IC-090
If you want the currently selected object to be set as the affected/owning object by default when creating a dimension, follow the steps below:
[1] Open [Options] in the [Menu].
[2] In the list on the left, select [Interaction], then scroll down to [3D Smart Dimension/Constraint].
[3] Check [Keep selected shape as affected/owning object when creating dimension].
[4] After checking the box, click [Apply] and then [OK].