With discretionary goals, workload management decides how best to run this work. Since workload management's prime responsibility is matching resources to work, discretionary goals are used best for the work for which you do not have a specific performance goal. For a service class with multiple performance periods, you can specify discretionary only as the goal in the last performance period.
Discretionary work is run using any system resources not required to meet the goals of other work. If certain types of other work are overachieving their goals, that work may be “capped” so that the resources may be diverted to run discretionary work. See Migration considerations for discretionary goal management for more information on the types of work that are eligible for resource donation, and how you may want to adjust those goals.