GD&T: GD&T (geometric dimensioning and tolerancing) is a standard symbolic language on engineering drawings that controls a feature's form, orientation and position, not only its size.
A plus-or-minus tolerance controls size. It does not say how flat a face must be, how square one face is to another, or exactly where a hole sits relative to the features it mates with. GD&T does. It uses a set of symbols in feature control frames, referenced to datums (chosen reference features), to define those requirements precisely.
Common controls include flatness, straightness, circularity, perpendicularity, parallelism, position and profile. Two main standards are in use: ASME Y14.5 in North America, and the ISO GPS family elsewhere. They are similar but not identical, so a drawing should say which applies.
Example: a plate has four holes for bolts that must line up with a mating part. Rather than tolerancing each hole’s X and Y distance with ± values, the drawing gives each hole a position tolerance of 0.2 mm relative to two datum edges. That describes the real requirement, and an inspector can check it consistently.
GD&T affects cost the same way any tolerance does: tight controls need more careful setup, inspection and sometimes better machines. A drawing full of tight geometric controls on every face signals a precision part, and the quote should reflect that.
A common confusion is that a 3D model alone carries GD&T. A plain STEP or STL does not, unless the model includes product manufacturing information (for example in STEP AP242). Usually a drawing is still needed. Read tolerances and cost for more.
Go deeper: How tolerances drive machining cost (and when to loosen them)