Form 15ca/cb mandatory?

Hi,

Quick question, is it mandatory to have form 15ca/cb when transferring fund from NRO to own NRE INR  account for the purpose of saving in NRE account. When I checked with bank apparently it is not mandatory if source of fund document can be provided like sale deed. Is it upto the bank what documents they need to use?. In the internet I see different information also about exception if the remittance does not required RBI approval.

Thanks