A node with a dataset-like icon with the name of a macro variable. When created, this node is listed with other avaiable datasets and dataviews in the process flow or project. When this "dataset" is assigned to a task the dataset defined in the macro variable is used in the sas code generated byt he task. If implemented as purely a macro variable with a dataset's name, then the linked-to dataset's indexes will available to the procedure statement.
One example of code that can't exist using EG and the Query Builder task:
In a way of speaking, its like a view of a dataset implemented in EG.