What is the purpose of the Default Subscription for Registered Users

Hello. I am using the latest version of the Membership plugin (3.5.1.2)

Under Membership > Options > General, can someone explain in detail what the “Default Subscription for Registered Users” does? I can choose a subscription or choose None. What does it means to choose None, and what does it mean to choose a specific subscription?

I know a person has to take 2 steps to purchase a membership:

1) Register for an account on the website.

2) Purchase the subscription via PayPal.

Let’s say a user registers for an account but does not decide to purchase a membership at that time, and decides to come back the next day to purchase the membership. If I set the “Default Subscription for Registered Users” to None, what does that mean for this user? On the other hand if I set the “Default Subscription for Registered Users” to a specific subscription, what does that mean for this user?

The user just wants to continue with his purchase. He already took step 1 (created his account), now he wants to continue and actually purchase the subscription. He would need to log in to his account he created the day before, correct? But if the “Default Subscription for Registered Users” is set to None, would this be possible? Or would the user need to create a brand new account to then purchase the subscription even though he created an account yesterday?

Or should I create a subscription specifically for the reason of choosing it for the “Default Subscription for Registered Users”?

Thank you.