I recently read a forum comment about an agent who, in response to a query letter, asked the poster for chapters and for "a business plan" for her novel. On a different site, a professional advisor/author/blogger type avers that some traditional publishers now ask fiction writers for proposals which will become the business plans for their novels. I'm an entrepreneur. I know business plans. But I don't know jack about writing one for a novel. Is this a new hill to climb?
My business plan for your novel is as follows:
1. Sell for huge advance to large publisher;
2. Sell zillions of copies
3. Sell film rights
4. Retire to France.
Oh, right, what's a business plan? Well, for those of you who don't know, it's a document that business owners use most often to acquire funding. Here's an example from the Small Business Administration.
When you look at it you'll see two categories that might not be familiar: revenue projections; and funding sources.
Here's the answer to your question:
Calling a marketing or publicity plan (which you still don't need at the query stage for a novel) a business plan is a sign the agent is throwing around jargon to sound important: an author can't project revenue any more than I can fly, and an author can't project funding any more than Jeff Somers can dance.
A business plan is used for companies who make products and sell to customers. Authors do not do that. If you're running your own publishing enterprise, sure, but if you're querying me, that's not your intent.
I have no idea why an agent would ask for a business plan unless they literally did not know what one was.