How does “Set first: the financial year” work on the Accounts settings page?

Every period picker, every comparative and every draft reads the year end from one place: Settings → Organisation → Financial year. It used to be a tab here as well, and the two drifted; the rail now links there instead. Set it before the first report, because 'this financial year', 'year to date' and the balance sheet's default month end all follow it. Where: Accounts settings (/accounts/settings).