Are the following definitions correct ?
Attention line is the part of the recipient address in a letter or on an envelope which names the person to whom the letter should be handed to.
Subject line is the part in a letter which refers to a particular subject, like a recent call and its date and something that has been discussed, like an order ect. The subject line can include a reference number of a process like an order or a project number.
I'm asking you to confirm the definition and its common use in English letters, especially in business.