What's the difference between a lawyer's letter and a letter of introduction when he meets a criminal suspect in custody?

Personally, I think there is no difference. However, in practice, the letter of introduction is generally used for meeting, and the official letter is used by the procuratorate for review and prosecution and court trial. If you really talk about the difference, it is: if you use a letter of introduction, you will use it once when you meet; If an official letter is used, it can be used for the end of the whole stage.