Currently, the ActivityDefinition element has a multiplicity of zero or one. This is directly reflected by the schema, 
stating <xsd:element ref="ai:ActivityDefinition" minOccurs="0">.

However, this is not entirely correct: while subsequent changes (and thus newly attached templates) of a given 
ActivityDefinition in "future" history records are optional, the initial entry is not.

This special aspect however cannot be expressed through the schema and should therefore be part of the normative text.