As I read what you wrote, my interpretation is about a bank letter. A pre-approved/pre-qualified bank letter is interchangable these days and both mean your loan "request" will need to go through a lengthy bank underwriting process before you are "officially" approved for funding. At any time during the underwriting process a bank can request additional docs and/or decline a request.
The majority of bank loan requests accompanied by "bank letters" are just requests. Until a bank's underwriting department completes its financial due diligence most sellers that received bank letters from a buyer know the deal could fall through at any time. Thanks for the post and best wishes to you.