The Related Row field specifies that the line’s code is tied to another line in the Line Definition. If the related row is suppressed - for example, due to zero suppression being set for that line – then the related row will be suppressed as well.
You do not need to enter brackets ([ ]) when entering the line code in the field and if you enter the code with surrounding brackets, they will be stripped off the value in the field when the Line Definition is saved.