This type of node is an alias of the Attributes type of node when inside a Grid (WPD) node. It was renamed in this context to avoid confusion with the Columns (grid) node, and describe its purpose more closely.
Summary nodes are used to include a section in the end of a grid that displays information related to the grid's content. For example, inside a summary node the sum of the values belonging to a particular column in the grid may be shown.
For more information regarding this node's properties, see the Attributes node documentation.
This is an example in which Tax and Total variables are placed inside a Summary Node.
Note that all variables placed in this node must be loaded manually.