The default schema used by content completion is the schema of the document type that matches the edited document. The list of document types available at contains a set of rules for associating a schema with the current document when no schema is explicitly specified within the document. The schema has one of following the types: XML Schema, XML Schema with embedded Schematron rules, Relax NG (XML syntax or compact syntax), Relax NG (XML syntax) with embedded Schematron rules, Schematron, DTD, NVDL.
The rules are applied in the order they appear in the table and take into account the local name of the root element, the default namespace and the file name of the document.
The editor is creating the content completion lists by analysing the specified schema and the current context (the position in the editor). If you change the schema you can observe that the list of tags to be inserted is changing.