Computers routinely round or truncate numbers when they perform numeric calculations and store the results. Consequently, small mathematical errors are introduced which may have a cascading effect. Solid modelers have analagous problems when performing complex calculations such as surface-surface intersection. The result is that gaps may appear between surfaces,neighboring surfaces may overlap, and relimited boundary curves may extend beyond the original surface definition. This is more likely to happen with loosely toleranced modelers than tightly toleranced modelers.
Tolerance refers to the values used by the modeler to determine coincidence and angle.
When modeling data is imported from a loosely toleranced modeler to a more tightly toleranced modeler like Helix Modeling, the problems inherent in the orginal data become visible and the system reports that the model is not valid. Helix Modeling does attempt to automatically heal the data or import. Nevertheless, some problems may remain, which can be fixed using Helix Craftsman.