A shaft is turned to Φ20 ±0.1mm. Is it straight? Is it round at both ends? Dimensional tolerance can't control this — you need form tolerances (straightness, roundness).
Two parts are machined with hole centers at 50±0.05mm and shaft centers at 50±0.05mm. They don't fit — because the centerlines aren't parallel. You need position tolerance (parallelism).
GD&T is function-driven — tolerance what matters for assembly and performance.
Straightness — how straight a line or axis is. Flatness — how flat a surface is. Roundness — how round a circular cross-section is. Cylindricity — how uniform an entire cylinder is (combination of roundness + straightness + taper).
Form tolerances are independent — they don't reference datums. A flatness of 0.1mm means the surface can't deviate more than 0.1mm from a perfect plane.
Position tolerances reference datums (A, B, C). Types: parallelism, perpendicularity, concentricity, position, symmetry, coaxiality, runout.
GB/T 1184-1996 defines standard tolerance grades. If no tolerance is indicated on the drawing, Grade H (general industrial) applies — it's not 'no requirement'.
What truly improved Chinese manufacturing precision isn't better workers — it's better drawings with proper GD&T callouts.