What is the role of the DB block in Siemens PLC? What is the type of data that can play a role in the program?

If it is a background data block, it is a bit troublesome, it needs to be changed in FB; if it is global, it is convenient: open the DB block, press the comment in the last line and press Enter. Then a new line will be created and the name will be entered. Can be entered arbitrarily), data type, comment (optional)