@set_anchor('MyLabel') ... please see @ref_anchor('MyLabel') for details.The label must be a string, and is currently required to be a valid identifier since it is used literally by the HTML weaver in an anchor tag. Latex imposes no such restrictions, nor does the plain text weaver. For HTML, the label is set as an anchor; for latex, the page number is given, for plain text the line number.
Note that inter-document (external) cross references are different to intra-document cross references. For truly external references to existing published works, use citations or bibliographic references. References across volumes of a work or project are not yet supported.