I think it refers to the deed transfer letter. The deed transfer letter is because the personal situation filled in the policy is abnormal or exceeds the consumption conditions of the general customers of the insurance company. The insurance company arranges the transfer personnel to find the customer for face-to-face investigation, and the notice given in advance is called the transfer letter.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.