Creo EZ Tolerance Analysis > General Workflow > Defining a Part Loop > Automatic Detection and Selection of the Parts in the Loop and Interacting Surfaces
  
Automatic Detection and Selection of the Parts in the Loop and Interacting Surfaces
In the New Stackup dialog box, under Assembly, if Assembly Constraints is selected, then at least one path from the assembly constraints is found between the two parts at either end of the stackup definition.
1. Select each of the paths. The parts involved in the loop except the other parts appear in the graphics window. The path represents the assembly constraints that are used for the stackup definition.
2. If none of the paths are correct, select Select to define the path manually.
3. Select the required path and click OK.
Defining a Part Loop