<xsd:sequence> required?

What I want to be able to say with the following piece of code is that 
one can use the mentioned elements any number of times and in any order.

   <xsd:complexType name="ModelType">
     <xsd:element ref="cellml:units" minOccurs="0 
maxOccurs="unbounded"/>
     <xsd:element ref="cellml:component" minOccurs="0"
       maxOccurs="unbounded" />
     <xsd:element ref="cellml:group" minOccurs="0"
       maxOccurs="unbounded"/>
     <xsd:element ref="cellml:connection" minOccurs="0"
       maxOccurs="unbounded" />
     <xsd:element ref="rdf:RDF" minOccurs="0" maxOccurs="unbounded" />
     <xsd:attribute ref="cellml:name" use="required" />
     <xsd:attribute ref="cmeta:id" use="optional" />
   </xsd:complexType>

Since the children elements can be in any order, the <xsd:sequence> 
element is not useful.  Neither are the <xsd:choice> and <xsd:all>.  Is 
it necessary that I use one of them in this case?  I do not quite 
understand the concept of a model group and have not made it all the way 
through Primer 1.  Thanks for your help!

Autumn A. Cuellar