WilmaRDF has two repositories defined and accessible from her Sesame Workbench application. She wants to perform a query against the combined contents of those two repositories. She doesn't want to have to manually combine the contents into a single repository. Perhaps they are large remote repositories, or are not under her control.
WilmaRDF goes to the “New Repository” page in Workbench, selects "Federation", and assigns an ID and Title. On the next page, she is able to make multiple selections from a list of defined repositories (in this case, just the two). She can also set some other parameters related to optimizing the federated queries, e.g., hints as to which name spaces to find where.
She then goes to the “Query” page, and queries her repository the same as any other.
If WilmaRDF likes to use the Console, she can also accomplish this task from the command line.