process instance id can be used to uniquely identify an instance. If you want to display this id in your form, you can do the following:
Define a variable at the process level in your BPMN xml.
In your task forms, use a display value or display text field to show this variable.
After the process instance is started, you can use a script task/execution listener to set this variable value to process instance id.
Eg: If your variable name is "processId", you can set it as execution.setVariable("processId",execution.getProcessInstanceId())
Cheers,