The 'Free Item' indicator is only used to facilitate the switching off of the 'Inv.Receipt' flag.
The flag UMSON is not contained in any table, only in structures.
But there is a correlation between the flag UMSON and the field EKPO-REPOS. So if you flag field EKPO-REPOS (an invoice is expected) the flag UMSON will be cleared and the other way around.

For further information about this flag: You will not be able to enter any conditions in your purchase order item (not even the delivery costs) once the flag for 'Free item' has been set.

In case the 'Inv.Receipt' flag is unticked, the system will not allow you to maintain any conditions in the purchase order, not even the delivery costs. In ME21N you can see that even the condition tab disappears as soon as you set the 'Free item' indicator or as soon as you de-select the 'Inv.Receipt' flag.

Whenever the 'Inv.Receipt' flag is not set, we are indicating to the system that all the goods are to be delivered free of charge. In other words, there can be no delivery costs associated with the PO. Also, no delivery costs are allowed without an invoice receipt.