Its a session option, when the informatica server performs incremental aggr. it passes new source data through the mapping and uses historical cache data to perform new aggregation calculations incrementally for performance we will use it.
When using incremental aggregation, you apply captured changes in the source to aggregate calculations in a session. If the source changes incrementally and you can capture changes, you can configure the session to process those changes. This allows the Integration Service to update the target incrementally, rather than forcing it to process the entire source and recalculate the same data each time you run the session.   _{Your Comment}