Also known as tolerance stacks, it is a mathematical and statistical problem solving method for variation accumulation allowed by the specified dimensions and tolerances in a design. It is useful in establishing and optimizing tolerances, an easy solution to make sure all parts in an assembly fit together. It is a useful design evaluation tool, that can also be helpful in troubleshooting manufacturing issues.
Tolerance stackups provide worst-case maximum or minimum values of dimensions and tolerances between two parts and it is used to calculate either a clearance or an interference. This analysis also provides statistical values based on the absolute arithmetic calculation combined with a statistical methods (such as Root Sum Square aka RSS, or Monte-Carlo) for establishing likelihood of obtaining the maximum and minimum values.