Usually, you will send documents (not letters) with a correspondence (letter) only to selected contacts or in connection with cases. Therefore, in a correspondence, documents are only sent as attachments. In this sense, a letter is not a document, but a correspondence. A correspondence is an activity you can create in contacts or cases, and access in your activity list. The correspondence itself is created from a correspondence template as a letter. All documents that are used as an attachment in the correspondence are registered in the correspondence record.
Attachments are also used in the context of activities such as tasks, appointments and memos. The documents that are assigned here are attachments.