You call it an "empty envelope" to emphasize that it should eventually contain a "letter" (perhaps a business letter) before you can send it by "post".

称其为“空envelope”是为了强调在通过“投递”发出它之前，它最终应该包含一封“信”(也许是商业信)。

youdao

You call it an "empty envelope" to emphasize that it should eventually contain a "letter" (perhaps a business letter) before you can send it by "post".

称其为“空envelope”是为了强调在通过“投递”发出它之前，它最终应该包含一封“信”(也许是商业信)。

youdao