Where is activiti:formKey="wf:activitiReviewTask" config evaluator?

[mangar]

I am running through Jeff Pots custom workflow tutorial.  I sort of understand

   <config evaluator="string-compare" condition="activiti$helloWorldUI">

and I can configure the share form the way I like.  When I click start workflow,  It goes to the User Task,  and defines the model to use

activiti:formKey="wf:activitiReviewTask">

My question is where the Share form for this task defined? And how does Alfresco know to use that form for this user task?

[sp2]

This form key is defined in workflowModel.xml