The provisioning system supports two types of plans:
Simple plans. A simple plan contains a collection of simple steps, but cannot call other plans.
Composite plans. A composite plan contains only other plans, called subplans.
The XML schema enforces the distinction between the two types of plans. Thus, you can use a top-level plan that contains calls to other subplans, or a simple plan that contains simple steps but no calls to subplans.
This distinction is important because the steps in a simple plan can only execute on the same set of target hosts, whereas the steps of a composite plan can execute on different sets of target hosts. A composite plan can use one set of target hosts for each simple plan that it contains.