Select a datum coordinate system from the movable instance and a datum coordinate system from the fixed instance.
When Abaqus/CAE prompts you to select the datum coordinate system from the fixed instance, you can select a datum coordinate system that was created in either the Part or Assembly module. In contrast, when you select the datum coordinate system from a movable part instance, you can only select a datum coordinate system that was created in the Part module.
Abaqus/CAE rotates the movable instance so that the selected coordinate systems are parallel.
If the parallel coordinate systems constraint conflicts with existing constraints, Abaqus/CAE displays an error message and cancels the operation. To avoid the conflict, you can try reversing the selection of the instance that will move and the instance that will remain fixed. Alternatively, you can convert the existing constraints to an absolute position and reapply the parallel coordinate systems constraint.