The attention line is used to indicate that the letter is supposed to be ready for any particular person or particular department.
Attention : Head of Marketing
This indicates that you do not know the name of the head of the marketing team and that the letter should be directed to the head of marketing.
Attention : Peter Hun – Marketing Department
This indicates that you know the name of the addressee, but not his/her designation. In that case, you should use the attention option. That means that the letter should be routed to the person Peter Hun who is in the marketing Department.
But these days this attention option is used to note that the core matter of the letters is for Mr. Peter Hun although the letter is addressed to the Head of the Marketing Department.
This option is used to draw the attention of the addressee to the core issue.