The easy answer to the question ‘What’s a Subject Line for’ is; to encourage a subscriber to open your marketing email. To support that, the way one judges a Subject Line is to compare the returns before and after a change. If the new line increases the open rate, it is better than the one it replaced.
There is one caveat with this though. The result of the test is only as good as the quality of the text. It is merely a comparative test, excellent in itself, but it is not a measure of how good the new Subject Line is. To construct a good, excellent or genius level Subject Line you have to understand the way it does its work.
What excites the reader sufficiently to make them open the email? Ask yourself what makes you pull a book from Waterstone’s shelves and read the blurb?
There are any number of reasons. It could be intriguing, or maybe it excites the imagination. It might be the subject matter; I regularly buy history books and am a glutton for anything on Pompeii. Then again, a humorous title is often enough for many to check the price. Enid Blyton’s Five on Brexit Island, not written by her of course, is merely the latest.
The book was directed at a certain group of people, almost a segmented email marketing list. The specifics included those who were old enough to read the series when new, or at least still in fashion, and not be put off by a parody of a favourite.
A wander along the fiction shelves will show a row of books all with what appeared to be identical titles, or Subject Line I suppose; Harry Potter. Closer inspection will reveal that they had different titles but the publisher thought that the name of the hero would sell more books.
Oddly enough, there was one that was different. It was the latest in the saga, a prequal, and predates our Harry. What was in largest type on that one was the author. J. K. Rowling sells, in more ways than one.
Searching for inspiration? Look on bookshelves.