Lifetime allowance enhancement factors provide a tax shield if you were affected by the introduction and changes made to the lifetime pension allowance - currently at £1,055,000.
You may be eligible for adjustments of your lifetime allowance limit upwards in case you are transferring a pension from abroad or you were employed overseas while contributing to your pension in the UK but you were not eligible for HMRC tax relief on your contributions to this pension at that time. Another scenario when you may be eligible is if you receive a pension credit following a divorce.
However, an individual has to apply at HMRC for these enhancement factors, they are not granted automatically. Therefore, you would probably know in case you hold them.
See also pension protection