I am looking at the Acme example.
The "Add to Bag" flow link is only visible when logged in. The page6.wr shows the following:
<Page xmlns:gr="http://www.webratio.com/2006/WebML/Graph" xmlns:layout="http://www.webratio.com/2006/WebML/Layout" id="mov1#mpkg2#hym5#page6" name="Combination" ignoreMasterPage="true" layout:pageLayout="WRDefault/Empty" gr:x="195" gr:y="5">
<ContentUnits>
<DataUnit id="mov1#mpkg2#hym5#page6#dau10" name="Details" gr:x="0" gr:y="0" entity="ent4" displayAttributes="ent4#att3 ent4#att4 ent4#att5 ent4#att6 ent4#att7 ent4#att20 ent4#att21 ent4#att22 ent4#att51">
<Selector id="mov1#mpkg2#hym5#page6#dau10#su11" defaultPolicy="fill" booleanOperator="and">
<KeyCondition id="mov1#mpkg2#hym5#page6#dau10#su11#kcond11" name="Combination" predicate="in" implied="false"/>
</Selector>
</DataUnit>
</ContentUnits>
<layout:Row>
<layout:Cell colspan="12">
<layout:Unit unitId="mov1#mpkg2#hym5#page6#dau10">
<layout:Attribute attribute="ent4#att3"/>
<layout:Attribute attribute="ent4#att4"/>
<layout:Attribute attribute="ent4#att5"/>
<layout:Attribute attribute="ent4#att6" layout:attributeVisibilityCondition="cexpr4"/>
<layout:Attribute attribute="ent4#att7"/>
<layout:Attribute attribute="ent4#att20"/>
<layout:Attribute attribute="ent4#att21"/>
<layout:Attribute attribute="ent4#att22"/>
<layout:Attribute attribute="ent4#att51"/>
</layout:Unit>
</layout:Cell>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
<layout:Cell/>
</layout:Row>
</layout:Grid>
</Page>
Where in the interface can a person modify layout:attributeVisibilityCondition="cexpr4".
I cannot find where this setting can be made through the interface.