You are in the right place in XML Spy - the help file has the following
with regards to options available to you re generating output.
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The DTD/Schema | Generate Sample XML File... command generates an XML file
based on the currently active schema (DTD or XML Schema) in the main window.
The options listed in the pop-up window are:
Generate non-mandatory attributes - Activating this option generates not
only mandatory attributes, but also the non-mandatory attributes, defined in
the schema.
Generate non-mandatory elements - Activating this option generates not only
mandatory, but also non-mandatory elements, defined in the schema.
Generate first choice of mandatory choice - Activating this option
generates/inserts the first choice of a mandatory choice.
Generate X elements if marked repeatable in Schema/DTD - Activating this
option generates the number of repeatable elements you enter in the text
box.
Fill elements and attributes with data - Activating this option inserts the
data type descriptors/values for the respective elements/attributes. For
example: Boolean = 1, xsd:string = string, Max/Min inclusive = the value
defined in the schema.
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You should also be aware that for a particular DTD/Schema it may not be
possible to generate all elements within a sinlge instance. For example if
you had
<!ELEMENT branch (x | y) >
then branch could contain 'x' or 'y' but never both in a single instance.