Instance opening script is executed for every new workflow instance when initiated. This script may be useful to assign dynamically changing default values. (ex: assign current date to form value)
In script you can use $Instance to get or set initiated instance properties, $Initiator variable specifies the who is initiated workflow instance but variable can be null for module type tasks. You can also refer to right panel to list available scripting objects.
Unlike task prework or postwork scripts $Xml variable attached to data model root element, not the pool element.