From Garzanti:

s.f. (non com.) castingoff, abandonment / (dir.) - di bandiera, cancellation from the register of ships (of a particular country) / (fin.) - di titoli, azioni, divestment of stock, shares.

Flagging in is the process of adding a vessel to the national registry and "flagging out" is the process of removing a vessel from a national registry. Generally flagging out refers to the practice of switching the vessel's registration to another country to fly operate it under a "flag of convenience". However, some vessels that have been deleted from national registries end up operating under no flag at all.