Re: What is ID.
In case of the bank:
For card present purchases the ID is possession of the card or possession of the card plus PIN.
For online purchases It's knowledge of a few short character strings from the card plus possession of a mobile phone whose mobile number corresponds with whatever the bank has currently been persuaded to accept as the account holder;s.
For online access to the account it's a user account ID, password and usually some form of PIN, possibly not the same as the card PIN, plus mobile phone as above. If you're lucky it's also knowledge of the answers to some security questions which the wise customer will ensure don't correspond with readily researchable facts.
In the olden days it was a face the customer's branch staff recognised. Tell that to kids these days and they'll not believe you.