Five layers, bottom to top:
- Target customer — who this is for
- Underserved needs — what they need that nobody serves well
- Value proposition — how you will serve those needs better
- Feature set — what you actually build
- User experience — how it feels to use
The bottom three are problem space. The top two are solution space. Fit is achieved when every layer genuinely supports the one above it, which is why a beautiful UX on a misidentified customer goes nowhere.
Work bottom-up. Diagnose failures the same way — a struggling product is usually broken lower than the layer people are arguing about.