In Lean thinking, the second principle is to map the value stream. This means charting every step involved in delivering a product or service—from initial idea through to the customer—and distinguishing which steps actually add value from those that don’t. Seeing the entire sequence helps reveal waste such as delays, unnecessary handoffs, and bottlenecks, so you can target improvements where they matter most. Mapping the value stream is the bridge between defining value and creating flow. You first define what the customer considers valuable; then you map the process to see all activities, so you can remove non-value-adding steps before you try to make the work flow smoothly or pull work through the system. After mapping, you would typically work on creating flow, then establishing a pull system, and finally pursuing perfection. Defining value is the starting point, not the second principle. Creating flow comes after you’ve mapped the value stream, and using a pull system follows once flow is improved.

Horizontal testing focuses on validating the system across its layers in a cross-cutting way. It proceeds in timeboxed cycles, testing the solution layer by layer and gradually adding more depth or complexity across the architecture. In practice, you might start by verifying how the UI interacts with services, then in the next timebox add the business logic and data interactions, and so on. The goal is to ensure each layer communicates correctly and to surface integration issues early, before delivering a full end-to-end feature. This aligns with the idea of taking a layer-by-layer, incremental approach rather than building a complete end-to-end feature all at once. The alternative that talks about delivering fully functional features across multiple layers describes end-to-end functionality (a vertical slice), not horizontal testing. Focusing only on the UI omits the cross-layer scope, and saying horizontal is the same as vertical is inaccurate.

In the Kano method, features are grouped by how they affect satisfaction: must-be (basics), one-dimensional (performance), and attractive or excitement (exciter) features. Exciter features, also known as wow features, are those that can trigger a strong positive reaction when present but don’t cause dissatisfaction when absent because customers didn’t expect them. That’s why describing them as features that evoke a high level of customer satisfaction is accurate—they’re the source of delight and differentiation. The other categories serve as baseline expectations or linearly increasing satisfaction with performance, so they don’t deliver the same standout boost as exciters.

Knowledge exists on a spectrum from explicit to tacit, with a middle ground called semi-tacit. Explicit knowledge is codified and written down—things you can share and transfer easily, like requirement specifications, models, or process diagrams. Tacit knowledge lives in people’s heads—their experience, intuition, and know-how that’s hard to articulate. Between these two is semi-tacit knowledge, which is not fully codified but is partially expressed in guides, templates, best-practice notes, or workflows that people can follow, though real mastery still relies on experience. This mix is why explicit, tacit, and semi-tacit is the best answer: it captures the full range of how knowledge can exist and be transferred in a project. The other options mix concepts that don’t describe how knowledge is expressed or shared—like ownership terms (public/private/shared) or presentation modes (formal/informal)—and one substitutes implicit for semi-tacit, which doesn’t clearly reflect the intermediate state between codified and deeply tacit knowledge.