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:This combination of electrodes can measure Z from five regions: both sides of the upper trunk (ZTRur and ZTRul), the middle trunk (ZTRm), and both sides of the lower trunk (ZTRlr and ZTRll) (13).
Category: