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:The vertical line marks the 6 kHz boundary that was arbitrarily set for manually distinguishing between flat and chevron syllables.
et al., 2008; Young et al., 2010; Wohr et al., 2011b) or stress level (Rupniak et al., 2000).
Category: