Section 22.5 (#TDbuild) currently says this regarding @source:
"If the source is specified in the form of a private URI, the form recommended is tei:x.y.z, where x.y.z indicates the version number, e.g. tei:1.2.1 for release 1.2.1 of the current TEI Guidelines. When such a URI is used, it will usually be necessary to translate it before such a file can be used in blind interchange."
However, tagdocs is meant to be generic so that they could be used for an ODD describing a non-TEI markup language. Therefore, we should make this generic and not imply that "tei:" should be used even if your ODD specifies a non-TEI markup language.