The <execSubplan> step has the following attributes:
planName – A required attribute of type entityName, which is the name of the plan to execute. When this step is run, a corresponding top-level <executionPlan> with this name must be specified. This name cannot reference an inline subplan.
planPath – An optional attribute of type pathReference, which is the path of the plan to execute. If this attribute is omitted, the path of the containing plan is assumed.
planVersion – An optional attribute of type Version, which is the version of the plan to execute. If this attribute is omitted, the latest version of the named plan is executed.