@arkaprabha-majumdar, you use rules when you want your assistant to do the same thing whenever it understands a certain intent. Stories should be used when your assistant’s next action depends on a sequence of events instead of a single condition(rule).
The way you define form using stories.yml on 2.x is something like this:
version: "2.0"
stories:
- story: form
steps:
- intent: user_name
- action: user_registration
- active_loop: user_registration
- active_loop: null
But, you can use rules if you want. In can check this post on Rasa’s blog about updating to Rasa 2.x.