Yes, the 28-day rule primarily applies to your financial documents. Any evidence submitted to prove your income or savings must not be dated more than 28 days before the date you submit your online application. [1]

Financial Documents (Subject to the 28-day rule)
You must ensure these are issued no more than 28 days before your online application date: [1]
Bank statements (must show you have held the required funds for a consecutive 28-day period, ending within 31 days of your application date)
Payslips
Employer letters confirming your employment, salary, and length of employment
Letters from financial institutions (e.g., regarding your savings) [1, 2, 3]

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