Certificates and endorsements
Seafarers hold Certificates of Competency (CoC) for their rank, supported by certificates of proficiency for specific tasks. When a vessel flies a flag different from the one that issued the CoC, the flag State issues a flag endorsement (a Certificate of Endorsement) recognizing that CoC, which is why flag-state recognition arrangements matter so much to crewing.
Watchkeeping and rest hours
STCW sets minimum rest for watchkeepers, at least 10 hours of rest in any 24-hour period and 77 hours in any 7-day period, with the rest split into no more than two periods. These limits overlap with the MLC's hours-of-work-and-rest provisions and are a frequent Port State Control focus, since fatigue is a recognized safety risk.
Why it matters for operators
A crew is only as deployable as its certification. Expired endorsements, missing proficiency certificates or rest-hour breaches can stop a vessel as surely as a technical defect. Keeping the crewing matrix current, and the documentation organized, is core to staying ready.