A Composite report brings saved subreports together through a common grouping. Build and test the component reports first. Their fields can come from different data sources, but the grouping must describe the same business dimension. Organization ID is safer than a displayed organization name because names can repeat or change.
Two numbers beside one another still need their own population definitions. Current headcount and September exits are compatible by organization, although one starts from Workers for HCM Reporting with All Active Workers and the other from All Workers. Keep an organization with only one component result rather than silently removing it. A composite doesn't repair a component that already excluded required workers. Run each component separately when a total seems surprising, then inspect the assembled output.