In the guidance (http://iatistandard.org/standard/activities) the attribute 'hierarchy' (on an iati-activity element) is defined as integer and specifies the level in the hierarchy of activities but in the XSD (http://iatistandard.org/downloads/iati-activities-schema.xsd) it is defined as a string and is described as a relative position as a parent (1) or child (2) - the distinction only matters in a hierarchy with more than 2 levels but needs clarification.
Alter the activity xsd schema from:
<xsd:attribute name="hierarchy" type="xsd:string" use="optional">
<xsd:attribute name="hierarchy" type="xsd:int" use="optional">
Proposal based on suggestion from Kit Wallace.