As implemented, the BDef and BMech objects are basically unchanged. Here's what the new CModel control object looks like:
DS-COMPOSITE-MODELdatastream specifies the structural requirements of member objects. The dsCompositeModel.xsd schema describes the expected format. For example, here's the DS-COMPOSITE-MODEL of info:fedora/fedora-system:ContentModel:
Pretty simple. It says, member objects must have at least these datastreams, and each be in the form specified. If multiple forms are listed in a single dsTypeModel, the datastream may be in any of those forms.