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:…did not use any chemical, biochemical or physiological characters, but in later taxomomic studies of Aspergillus physiological tests (Klich
& Pitt 1988) and secondary metabolites (for example Frisvad 1989; Frisvad et al. 1998a, 2004; Samson et al. 2004; Frisvad et al. 2007) have been introduced.
Category: