Flattermarken are marks printed in book binding and print finishing, where they help check that folded sheets are gathered in the right order. Each digit is a single bar, and there's no text.

What it holds
Characters	Digits only. A 0 draws no mark, so a value of zeros alone draws nothing.
Length	Up to 90 digits.
Check digit	None.
Readable text	None: Text position isn't offered.

The rest of the settings - quiet zone, color - are in the Barcode section.

What goes wrong
Numeric only data expected.	The value has something other than digits.
Input data too long.	More than 90 digits.