Should @type be documented as an optional attribute for <relatedItem>? The Guidelines say "@type attribute should be used" and indeed all the examples do so.
@type is already optional on <relatedItem> since it's a member of att.typed. You were perhaps confused because http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-relatedItem.html lists @target, and you might have thought that the element specification defined only this attribute for this element. But just above this attribute definition you'll find various attribute classes of which <relatedItem> is a member. The element inherits attributes from all of these classes.
John P. McCaskey
Yep. My mistake. It was the emphasis in the Guidelines that threw me.