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:We can distinguish two main models [6]: instance selection as a method for prototype selection for algorithms based on prototypes (such as knearest neighbors) and instance selection to obtain the training set, for a learning algorithm that uses a training set (such as classification trees or neural networks).
Category: