You use networks and internal activities in your project.

Capacity requirements are the demand for capacities that result from planned or actual orders. If a work center is overloaded, it means that its capacity requirements exceed its available capacities. To reduce the overload of the work center, you can trigger automatic capacity leveling inside the project planning board, which is a tool that allows you to plan and monitor capacities for network activities. You can also shift activities backward or forward on the time axis where more capacities are available, which means changing their start and finish dates manually or automatically. Reference:https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html