It's not the beginning of a section that matters, it's how it ends. Each section should end with an event from which there is no going back. From this point the story has no choice but to move on. Examples would be...
Somebody dying - there's not much chance of going back there.
A married woman having sex with a man not her husband - she can never claim to be faithful again.
A revelation - "I am your mother!"
A realisation - I don't have to stand for this any more.
A complete change of location - In A Single to Rome, Natalie relocates to (there's a clue in the title) Rome at the end of Section A.
I recommend that writers have an idea of what at least 3 of their section ends are going to be before they start writing. That will provide the novel's basic structure and make it easier for you to write as it breaks the novel into more manageable chunks. 3 section ends = 4 chunks of writing (the final section end will, of course, be The End). If your novel is aiming for 100,000 words, that's 25,000 per chunk. 4 section ends, and it's 20,000 each. It's much easier to think of 20,000-25,000 words in one go than it is 100,000.
The starting point for the sections is 3 Act Structure. Each act ends with a turning point (my 'you can't go back'), with Act II split into 2 sections with a turning point in the middle.
Act II Part I
Act II Part II
I'm a fan of 3 Act structure because I think it's a useful tool to help you write a novel, but it's easy to get hung up on the academic aspects. Calling the acts sections helps to keep it more fluid, less technical. Besides, you may end up with more than 3 turning points - I ended up with 4 in A Single to Rome, so technically it has 5 Acts (or a 3 part Act II).
But I'm only interested in structure when it actively helps writers and because every writer is different I don't think it's helpful to be too prescriptive - which is why I'm not a fan of The Hero's Journey.
An outline and thinking of a novel in sections help me. Hope they help you too.