Foundations — Requirements — functional vs non-functional, user stories, acceptance criteria

Before you can read the parent note, you must own every word and squiggle it uses. This page collects every symbol, term, and notation there and builds each from zero: plain meaning → the picture → why the topic needs it. Read top to bottom; each block leans on the one above.

Parent: Requirements (parent topic).

0. The very first idea: "system", "stakeholder", "requirement"

Three plain words appear everywhere, so we pin them down first.

Look at the box above. Arrows go in (what the user provides) and out (what the system gives back). A requirement is a label we stick on this diagram saying "this arrow must exist" or "this arrow must arrive within 2 seconds". Keep this box in your head — every later idea decorates it.

1. The symbols in the cost formula

The parent note opens with:

$\text{Cost}(t)\approx C_0\cdot k^t,k>1$

That looks scary if you've never met it. Let's earn every piece.

Look at the orange curve: from stage $0$ to stage $4$ it barely moves at first, then rockets. If $C_0=1$ and $k=10$, then stage $4$ costs $10^4=10,000$. Why the topic needs this: it is the whole reason requirements exist — writing them is the cheap left-hand side of this curve.

2. The user-story template symbols: $⟨\cdot ⟩$ and "shall"

The parent writes:

$\text{As a }⟨role⟩,\text{I want }⟨goal⟩,\text{so that }⟨benefit⟩.$

3. The Given–When–Then symbols

$\underset{\text{context/state}}{\underbrace{\text{Given}}}\underset{\text{action}}{\underbrace{\text{When}}}\underset{\text{expected outcome}}{\underbrace{\text{Then}}}$

Every arrow in this figure is one part of the test. Why the topic needs it: it converts a vague story into a machine-checkable pass/fail — the definition of "done".

4. Thresholds and comparison symbols: $<$, $\le$, "95th-percentile", "£", "%"

The NFR examples use numbers with units. These symbols carry the whole meaning of "measurable".

The figure shows a spread of response times with the 95th-percentile line drawn in. Why the topic needs it: acceptance criteria for non-functional requirements only become testable once a symbol like $\le$ meets a number and a unit.

5. The acronyms as compressed checklists

These aren't maths, but they are notation — memory-shorthand you must expand on sight.

6. Where each foundation feeds the topic

Read it downward: the system box makes the idea of a requirement possible; requirements split into functional and non-functional; functional needs get packaged as user stories; every story is nailed down by acceptance criteria, which borrow Given–When–Then and the threshold symbols. The exponential cost curve is the motivation sitting behind all of it. This whole pipeline is the front end of the Software Development Life Cycle and drives Agile and Scrum and Software Design.

Equipment checklist

Cover the right side and test yourself. You are ready for the parent note when every line is instant.