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:For definition of infection we used criteria provided by the Guideline for Prevention of Surgical Site Infection (SSI), which distinguishes between superficial incisional SSI, deep incisional SSI and organ/space SSI [13].
Category: