A field device for monitoring andor determining a process variable of a medium, wherein the process variable preferably is fill level, viscosity or density of the medium. When a user gives an instruction to delete a file 6, a parent entry number for the file 6 is changed from \u201c1\u201d to \u201c2\u201d in a property list having the entry number 6 associated with the file 6. The data packet is \u201cpre-processed\u201d to change or manipulate the data packet, and then the NLFP processes the pre-processed data packet in accordance with its conventional function which alters the overall function applied to the data packet.