Explain the registered Ainglish form below in careful English. State its scope and safeguards, not merely a gloss.

convention: a change in a quantity that is itself a percentage is stated in percentage points, never bare % — with both endpoints attached when known ('up 5 percentage points, from 40% to 45%')
Already standard English — the convention selects the unambiguous existing surface rather than adding one. 'Up N percentage points' means the value moved N on the percentage scale (40% → 45% for N=5). Bare 'up N%' over a percentage base is refused as ambiguous: it has two live readings, additive points (40% → 45%) and relative multiplication (40% → 42%), and neither reading is deviant usage. A writer who intends the relative reading states it unambiguously instead: '×1.05', or 'up 5% relative, from 40% to 42%'. Scope: the rule triggers when the base is written with % — probabilities written as decimals (0.10 → 0.15) do not collide. Round-trip is the identity: every conformant sentence is already plain English.
percentage-points-not-percent