CATIA 3D Functional Tolerancing and Annotation

http://www.cadfamily.com/downinfo/301272.html

Parts cannot be manufactured to an exact sizes because of natural imperfections in the
world, including machine tools, part programs, tooling, and also human errors. So a plan is
needed to allow the production process to accept the imperfections. This gave rise to the
concept of Tolerancing or Allowable Deviation.
A tolerance is the amount of deviation from the exact size, allowed on a part. Any part within
the tolerance will still be functional.
Designers create parts that are generally
required to create assembly of a particular
product.Each part will be engineered to
perform a function and, most importantly, to
assemble with a mating part.
Thus every part should EXACTLY FIT in the
final assembly and answer the requested
functions.