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:Rail clamps that have used the nonbreakdown approach can further be classified into single time-constant-based [1]–[6], dual time-constant-based [7]–[9], and latch or feedbackbased [10]–[15] rail clamps.
Category: