F4211 and F42119

I understand that when Sales Order is entered , rows are added to F4211 for each line item on the order. When the sales order update program is run, completed line item s are moved from f4211 to F42119.

But there is a comment in the documentation which I am unable to understand:

"It is possible to have different line item for the same order in both F4211 anf F42119 and it is not possible to join F4211 and F42119"

Can somebody elaborate this with an example.

Thanks