The difference between product plans and business plans
Once you seriously start working on your business plan, the most common shortcut I see with technical entrepreneurs, which doesn't work, is to use their product plan with a few changes as the business plan. The result is a disaster, as these may sound similar but are not even close to being the same thing. I define a product plan as a detailed description of your product or service, with a bit of business thrown in at the end. A business plan is a detailed description of your business, with a bit of product description thrown in near the front.
Don't get me wrong. It's definitely positive to have a product plan or specification. A simple differentiation is that a product plan is designed for internal use, to get the product out. A business plan is an "outward facing" document for external investors or for C-level executives within your own company.
The product plan tells your developers what to build and the marketing team what to market...