Ask HN: What are the datamatrix barcodes on letters?

I can't seem to find a satisfactory answer on google. Whenever I receive a bank statement, letter from my doctor, or utility bill, there are usually one or more Datamatrix 2D barcodes on it in varying positions on the page (where they would not be visible to the courier). What are these barcodes? Are they coming from some common software package used for content management? Are they used by a company that prints and sends letters?

They seem to be present on mail I have received in both the UK and US.