It's thermal ink, the whole receipt is covered in it so there's no waste of ink. That machine probably stores the data digitally in a computer through one of those cables and uses it to keep track of what and how many ingredients and products are left, appart of keeping track of earnings too. In order to do so, it has to phisically print a receipt so I guess that the lady probably has been half a year with nobody asking for the receipts and decided to do this to not waste more paper. PS: That paper is toxic and contains carcinogen substances thanks to the ink so it's a good practice to stop using them if possible