Merchant Mariner's Document

Under the Seafarers' Identity Documents Convention, 1958, countries with a merchant navy (also called a merchant marine) require identifying credentials for their mariners. The Merchant Mariner's Document (MMD) or Z-card in the United States, and the Ordinary Seaman's Certificate in the United Kingdom are examples of these credentials.

Source: Wikipedia — Merchant Mariner's Document (CC BY-SA 4.0)