There are lots of different types of editor in a publishing company. At least there are several different editing roles, though in a small publishing house an editor may double up, performing more than one role. The commissioning editor is the one who - guess what? - commissions books. To you, the author, the commissioning editor is pretty close to god.
The commissioning editor builds the list. That's not writing down things to do that then never get done, like our own lists. It's thinking up and acquiring books that make a collection with coherence and integrity. Sounds posh, doesn't it? The commissioning editor commissions books either by deciding they want a certain book and looking for an author to write it, or by looking at proposals that come in and picking from them.
Your job as writer (with the help of your agent if you have one) is to identify the commissioning editors of lists into which your book would fit. There is no point in sending a proposal to a commissioning editor who does not have a list that it would suit. I can't stress that strongly enough. An editor is not likely to start a list just for your book. Nor are they likely to take a book that would compete directly with one already in their list (or in the list of another editor in the same publishing house).
If you are happy to write books to commission, rather than coming up with the ideas yourself first, you can contact commissioning editors of relevant lists and introduce yourself, explaining what you write. This only works for some types of books. Don't approach an editor at Faber saying you'd like to write some literary novels and would they get in touch, please, when they need some written? That doesn't happen. But if you want to write reading-scheme-type fiction for early readers, or books on cat care, or travel guides, you can approach a commissioning editor, with examples of your previous relevant publications, and ask if they have any openings in their list. Expect to be told 'No'. That's the default answer. You just might get lucky. Then they may say 'we're commissioning short stories with a scientific content supporting key stage 1 science' or some such. And then you decide if you can do it and want to do it.
The names of commissioning editors - especially those who are good to work with - are sometimes a closely guarded secret. Most commissioning editors are drowning under a deluge of unsolicited proposals and manuscripts (slush pile). To ask an author the name of their editor with the intention of sending them your own proposal or manuscript, especially if you are unpublished, is not polite. An editor will take more notice of something sent in with a name check: 'Stroppy Author gave me your name' or whatever. It suggests an endorsement from a writer they trust. We will not give those endorsements to people whose work we do not know (and consider good). So if you meet me at a party and I won't tell you the name of my editors, that's why. You can find the names of editors in the Writer's and Artist's Yearbook - or at least the name of the person to send your proposals to, which may or may not be the real commissioning editor.
If you phone a publishing company and ask the name of the commissioning editor for the list you are interested in, you may or may not be given it. I've been refused the email address of a commissioning editor by a telephone-answering Hitler. Even when I told her I'd already published more than 50 books of the same type, she was reluctant to give it to me. Funny way to run a business, employing staff to make it difficult for the buyers to acquire the product they sell. But then, publishing is a funny business.