DITA allows cross references to <topic> (including each basic type), <section> (including <example> and <refsyn>), <table>, <fig>, <fn>, and <li>. No other elements. DITA2Go provides an ID for each instance of each of these elements, if a suitable ID is not already present.
When a <xref> tag appears in a context where it is not valid, such as in a title, DITA2Go automatically wraps the <xref> in a <ph> element, and assigns an outputclass attribute to the wrapper; see §24.9.2 Specifying an outputclass for cross-reference wrappers.
To provide a link destination for target elements that do not already contain a DITAElemID PI marker (see §24.4.6.1 Specifying a value for the id attribute), DITA2Go makes the ID of the target element the content of the first HyperJump PI marker in the element; or, in the absence of HyperJump PI markers, the numeric ID of the cross-reference marker.