If you have selected this option, all tags you enter are closed automatically and you also will be prompted for element names, attribute keys and attribute values.
Tag completion lists all known child elements of the parent element, without regard for further constraints expressed in the DTD/XML Schema/RELAX NG grammar. The set of known child elements is derived from the document itself and updated every time the document is opened or saved, so tag completion will work even if a document is not attached to a schema.