When creating aggregates, extensions are merged into the resulting aggregate feature. There are default rules for aggregating extension content, which essentially is appending all the extension content of a given type. However custom merge plugins can also be provided. After the merge a postprocessor is always run which can perform additional operations based on the extension content. Note that both the aggregate task of the slingfeature-maven-plugin as well as the launcher perform merge operations on all the feature models these are provided with.
TBD