Why are we requesting customer details in MonoConfiguration?
Customer detail is to uniquely tie the user to the account they're linking as an extra security layer.

What is mono customer ID for Existing customers and how do we get it?
The customer ID can be gotten from the customer endpoint or the dashboard. You only need it for one-time and recurring payments. It's not necessary for existing data customers.
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Why do we need confidential details for new customers like name, email, identity type and identity number?
The ID type and number are only required for creating mandates. We only need the customer's name and email for data and one-time payments. This is to identify the customer who is linking their account again uniquely.