The National Average Earnings (NAE) preference is used as the default rate of income inflation for Employment and Other Income.
NAE is also used to escalate future UK State Pension benefits as part of the Triple Lock formula, whereby State Pension is escalated by the higher of NAE, CPI or 2.5% per annum.
This default setting can be overridden for any individual Employment earnings or Other Income in the plan by adjusting the relevant values under Advanced Settings > Growth (on the Employment screen) or Income Growth panel (on the Other Income screen).
Plan Preferences vs. System Preferences
Changes to this setting on the mirror Default Inflation / Growth Rates panel in System Preferences, on the left side of the screen, will be used only going forward, as you create new client cases. System Preferences are used as a template only for new client cases. Changes to System Preferences will not retroactively affect your existing client cases.
If you want to change this preference in any existing client cases, you will need to open and edit them individually, in each case's Plan Preferences.