Contains the API of the core elements like data cells, data tables, nodes, and all elements the infrastructure of the flow is based on. In the subpackage data you'll find all definitions of elements needed to store and access the actual data (the data that is transported through the different nodes in the flow and being analyzed, manipulated, mined, etc.). These are DataCells (with their different flavours like DoubleCells, IntCell, etc.), the interface definitions for the DataTable (which carries all attribute vectors) and related classes. There are also default implementations for these data containers, which are all simple objects storing the data in memory. The other big subpackage is node. It contains all classes needed to define the infrastructure of the dataflow. The most important ones are the Node, which uses the abstract classes NodeModel, NodeDialog, and NodeView. These will be implemented by a designer of a new node for the workflow.
You also find an implementation of a default workflow manager in the node package. It is an object that allows for defining flows (by adding nodes and connections), and executing the nodes in parallel. The util subpackage contains all kinds of small utility classes that are useful in many nodes.