The documentation is unclear for the param element variant which contains xs:string content. This variant can appear as a child of either bind_material or accessor. It is multiply documented, both under param (data flow) and under param (reference). The former doc does not mention the fact that xs:string content is possible (it is, according to the schema). The latter does mention it, saying it "represents the SID of an existing parameter". I am unsure what the semantics of this would be in either bind_material or accessor context. The examples given inline in the doc do not show any xs:string content (in any context).