To best illustrate this feature open the sample template "Variable Invoice Sample - Template.docx" found in your template samples directory after installing the Designer here %USERPROFILE%\Documents\Windward\templates

This template uses an input parameter to decide at runtime which of various orders to pull the related list of products from. As you change the input parameter you also change the output.

If I run this template for order number 10249 I will get a list of two products associated with that order so the Data Count button returns 2 rows as it's answer when the runtime variable is 10249

If I run this template for order number 10251 I will get a list of three products associated with that order so the Data Count button returns 3 rows as it's answer when the runtime variable is 10251