The difference between a proposal and a letter

Different forms, different contents, etc.

Different forms: Letters are an applied style and a common communication tool. A proposal is a written article drawn up by an organization or society, which puts forward suggestions to the society or suggests that the members of the society do something together.

Different contents: the contents of the letter include several parts, such as title, body, signature and date. The contents of the proposal include the title, the title and other parts.