Supplier Order "billed" column / status

Hi,
on the list page of supplier orders we have the column “Billed” which is a row in the table ‘llx_commande_fournisseur’
This value is always 0 and I do not see when this is triggered to be changed to 1.

We want to filter for this flag but when we create a “supplier bill” directly from the “supplier order” there is no change of this value for the connected supplier order.

Is this intentional? or is that a bug?