In chapter 5 the example for the lobby is like following:

<obj href="obix:Lobby">
<ref name="about" is="obix:About"/>
<op name="batch" in="obix:BatchIn" out="obix:BatchOut"/>
<ref name="watchService" is="obix:WatchService"/>
<list name=”models” of=”obix:uri” null=”true”/>
<list name=”encodings” of=”obix:str” null=”true”/>
<list name=”bindings” of=”obix:str” null=”true”/>
</obj>

"ref" is defined in 4.2.3 as "A ref element MUST always specify an href attribute. A ref element SHOULD specify the type of the referenced object using the is attribute."

The reference to "about" and "watchService" is missing the "href"