This is my work-in-progress understanding of what the SOAP data model amounts to; it doesn't reflect the views of the XML Protocol WG, and may quite likely not reflect the reality of the SOAP 1.2 spec either. Consider it my 'learning about SOAP in public' exercise...
It is couched in terms of candidate text for the 'soap data model' bit of the spec, though I'm well aware it's too rough to be considered for that.
The SOAP data model is a subset of the Directed Labeled Graph (DLG) class of data models. As such it provides a simple type system that is a generalization of common features found in type systems in programming languages, databases and semi-structured data. Edges in the graph are labelled with simple ("locally scoped") string values or ("globally scoped") URI-qualified names corresponding to the XML namespace mechanism. Nodes in the graph are either simple (datatyped) values or represent complex objects that are described by an aggregation of relations (labeled edges) to other values.
The SOAP data model abstracts away from the details of the referencing mechanisms (such as XML ID/IDREF, URI, XML element containment) that support any particular concrete encoding of a SOAP data graph. SOAP data model instances, regardless of encoding format, can be merged to aggregate the encoded information by folding together the abstract edge-labeled data graphs. Globally scoped (URI-named) edges and nodes provide one of many strategies to support SOAP data model merging.
The SOAP data model provides an abstract view of XML-encodable data graphs, represented as an unordered set of node-edge-node 3-tuples. XML serializations of the SOAP data model are by necessity ordered. All concrete XML encodings of the SOAP data model must therefore specify the significance of document (Infoset) ordering with respect to the abstract edge-labeled graph model. Within a compound value (complex node), each relation to another node is potentially distinguished by a role name, ordinal or both. In this latter case, the edge name itselfs corresponds to a pair of ordinal and role name. (@@hmm, not sure here...)
The SOAP data model is consistent with, but does not specify, a number of mechanisms for describing a "DLG Schema" corresponding to meta-information about node and edges types in the SOAP data graph. UML, RDF and other representational formalisms may provide additional information about the node and edge types that occur in a SOAP data model instance. SOAP does not mandate the use of any particular mechanism for this. DLG-schema information can be used to support the concrete serialization of SOAP data graphs into XML, for example by providing information about cardinality or domain and range constraints on (globally scoped) edge types.
[...etc. lots of material from SOAP Encoding spec missing here]
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