Parts cannot be constrained to parts or features that are located farther down the tree.
Parts can be reordered by dragging them to a different location in the tree, subject to limitations of existing constraints.
One can also make a feature early in the model tree that represents component placements so that there is less need to make references between parts, but this method requires more care because not carefully matching the feature to the parts can allow the assembly model to fail to represent the actual assembly.
It is helpful to think of Creo Parametric as a procedural modeler - at any point in the process, a part or feature can only refer to items that have already been evaluated or regenerated earlier in the procedure.
