ID Generator Field is used to generate a running number based on Environment variable. Please follow on to Generate Reference ID on the usage.
Figure 1: ID Generator Field
Figure 2: ID Generator Field Properties
ID: Element ID
Label: Element Label
Environment Variable Name: Name of the Environment Variable to be used to keep the running number.
Format: Determines the pattern of the generated value. (E.g. "REF-?????" would yields "REF-00001")
Hidden: Determines if this element should be made visible in the form.
One may map the generated value of this element to a Workflow Variable.
Read more about Workflow Variables.