What the confirmation is for
If you move into a home in Germany, you must register with the residents' registration office (Bürgeramt) within two weeks (section 17 (1) BMG). To do this, the registration authority needs confirmation from your landlord (Wohnungsgeber) that the move-in actually took place. Without this document, registration is usually not possible.
Who issues it
The landlord (Wohnungsgeber) is usually whoever lets you use the flat: for a rented flat, that's the landlord; for a sublet, the main tenant; for owner-occupied property, the owner themselves. The landlord can also authorise someone else to issue it, for example a property management company. If the landlord is not the owner, the owner's name must also be stated.
These details are mandatory
Under section 19 (3) BMG, the confirmation must contain the name and address of the landlord, the name of the owner if that is a different person, the move-in date, the address of the flat, and the names of the people registering. The generator asks for exactly these fields and checks before download that all of them are filled in.
Deadline and fine
The landlord must confirm the move-in within the two-week registration deadline. Anyone who fails to do so, or does so too late, risks a fine of up to 1,000 euros. Anyone who provides an address for a fake registration, even though no one actually lives there, risks up to 50,000 euros (section 54 BMG).