A Render Pass can inherit attributes from a parent Render Pass. The parent-child relation is defined through a connection on the child pass’ "parentRenderPass" message attribute. For example, the following MEL command will set ‘parentPass’ as the parent of ‘childPass’:
DRP_setParentPass("childPass", "parentPass");
When looking for Render Pass attribute values, 3Delight for Maya starts by looking in the Render Pass that is being rendered, and will then search this pass’ parents for the attribute presence. If an attribute is not found anywhere in the hierarchy, its default value will be used.