A bank debit memo could be a charge for interest owed to the bank, a loan payment, a fee owed for the printing of checks, a fee for the handling of a check that was returned because of insufficient funds, a transfer of funds from the bank account to another account at the bank, and so on.
The charge, decrease, or reduction is likely called a debit memo because the checking account balance is a liability on the bank's books. This is the case because the bank has your money as one of its assets and it has your account balance as one of its liabilities. When the bank decreases your account balance, it is reducing its liability. Liabilities are reduced with a debit entry. That also explains why the bank credits your account when your account balance is increased.
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