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:By default, iMetAMOS uses the k-mer based Kraken [51] tool, but the alternative methods of FCP [52], PhyloSift [53], PHMMER [54], and PhymmBL [55] are also supported.
Category: