Dependencies
When you export content, not only the items you select are exported, but (by default) also the items on which those selected items depend: the selected items' dependencies.
For example, exporting a Component also exports its Schema. Dependent items may already be present on the destination Content Manager. For example, if you are updating a set of Components, you may not need to export the Schema along with it, because you transferred the Schema earlier and it has not changed since then. In this case, you can filter the Component-Schema dependency to speed up your transfer.