These functions are available for use in the $group stage, accumulators are operators that maintain their state (e.g. totals, maximums, minimums, and related data) as information progress through the pipeline.
When used as accumulators in the $group stage, these operators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.
Some operators that are available as accumulators for the $group stage are also available for use in other. When used in these other stages, these operators do not maintain their state and can take as input either a single argument or multiple arguments.