On the far left we have category romance, where the romance is the only thing the author and the reader, presumably, is interested in. Category romance specifically does not want sub-plots and sub-characters running off and doing interesting things, taking interest away from the hero and heroine. The author must focus on the couple in question. These days, interest does not stop at the bedroom door. More and more blow-by-blow encounters are detailed inside the bedroom - or the equivalent.
The other extreme is of course the literary end. These books are often three and four times longer and detail all sorts of other things beside the central romance - if there is one. C J Sansom manages to write almost 450 pages without a central romance featuring at all and I love his books. Cornwell's Sharpe has a few stabs at romance but there is so much more about daring-do, war and skullduggery. Writers like Forester, Clements, Winston Graham, Mitchell and Gabaldon set their characters in a particular time period and then weave them into the history. Writers like Parris and Clements centre on a murder mystery.
This where the lines blur. Readers will put authors in differing places on the line. Some will say Gabaldon is literary because she has great swathes about the American War of Independence in her Outlander series. So did Mitchell in Gone with the Wind, but in both those books, the central theme is the love affair between Claire and Jamie, and Scarlett and Rhett. We could be very analytical about it and put every title on a sliding scale of romance v literary-ness, but who has the time? Certainly not me! It is a task for each reader according to their personal taste, should they chose to do it.
The other thing that affects the argument is the male-female reading bias. In general terms, though not everyone fits into these divisions, men like action, and women like romance. Men like tighter writing, women want feelings explained. Men's reviews still seem to have more kudos than those written by women. Men, of course, review the Sansom, Forester, Cornwell "serious" type of historical novel. Perhaps they write better reviews? I don't think I've seen this type of historical novel reviewed by a woman, but they must be, surely? If not, they ought to be.
Perhaps Byron had the answer when he said "Man's love is of man's life a thing apart; it is a woman's whole existence." Follow that through and you have an answer to the basic question, though you may not like it.
As it happens I've got the rights back for a book I placed with MuseItUp Publising and have begun editing it prior to publishing on Kindle. I always thought it was priced a little too high with Muse,and that it would sell better if I "Kindled" it. I began last night, and right away I can see the difference in style - Reluctance, soon to be known as THE GYBFORD AFFAIR, is definitely historical romance. THE QUEEN'S COURIER is more about the times and how they affect people's lives. So, I have my answer.