[SINCE Orbeon Forms 2017.1]
A dataset is a mutable set of data, usually returned by a service and stored in the form in XML format.
A dataset is transient at this point:
It is not saved alongside the data in the database.
It is not passed when navigating between the "edit" and "view" modes.
Instead, a dataset lives as long as the user stays on the given form page.
You create a dataset using the Actions Editor. The data returned by the service is stored into the dataset in XML format.
A dataset is identified by a name. Multiple actions calling services can store data into the same dataset. The action which last updates the dataset overwrites the entire content of the dataset.
Storing data into the dataset only makes sense if you make use of that data at a later point.
Datasets do not work in section templates yet, see issue #3132.