I took a copy of this document to my CM BBL branch to speak with the manager about it. I also brought with me my fix deposit passbook which shows a minimum of 500K baht deposited for the last 6 ½ years. My extensions of stay over the past many years is based on marriage to a Thai national.
The manager seeing the document acknowledged it was in effect and said it was an IMM requirement now to maintain, in the case of an extension of stay (EOS) based on family/marriage, to maintain the minimum balance of 400K for 4 months after the bank provided a bank letter confirming the applicant's deposit. I responded that IMM regulations only required the minimum balance of 400K being deposited with a bank 2 months prior to applying for an EOS and that the applicant was not required to maintain any deposit after receiving the IMM stamp extending their EOS in their passport.
The manager asked if I had my bankbook with me and I gave it to her...scanning the multiple entries in the nearly 7 years I've had this fix deposit account solely for EOS purposes, she told me not to be concerned and I would not be required to sign this document in the future. She explained it principally was for applicants who had just opened a bank account for purposes of obtaining their EOS as many individuals were closing these accounts as soon as their EOS had been obtained. I believe the requirement for being asked to sign such a document derive from BBL endeavoring to recover their costs to open these accounts for new customers who may close them so soon after opening one...not that the IMM requires this but to reduce BBL's costs of doing this business.
I would suggest that anyone that may be concerned about this document as it may or may not apply to them, is to go speak with your BBL branch manager before the time you wish to get their bank letter confirming the necessary funds.