Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Because of their controllable and reversible transition, ER fluids have been extensively applied for a wide range of electromechanical engineering devices such as isolation dampers, shock absorbers, and clutches as well as for engine efficiency [9–11].
Category: