If you are installing a z/OS ServerPac or Product ServerPac order . . .

If you install your ServerPac or Product ServerPac order shortly after it arrives, you should not need to install additional RSU service. It should already be included in your order.

But if more than a few weeks have passed since your order arrived:

Consult ServerPac: Installing Your Order, in the topic about service included in your order, to find the service level of the system as shipped.
Order a service package using the service delivery vehicle you prefer, such as ShopzSeries, SMP/E Internet Service Retrieval, or RefreshPac. Use a starting service level one month later than the level identified in ServerPac: Installing Your Order.