MMC matters when you want assembly protection with bonus tolerance. Example: a clearance hole is Ø10.00 to Ø10.10 mm, and you specify position Ø0.20 at MMC. The hole MMC size is 10.00 mm. If the produced hole measures 10.06 mm, the bonus tolerance is 0.06 mm, so the total allowed position becomes 0.20 + 0.06 = 0.26 mm. That extra allowance helps the pin still assemble even when location drifts, because the hole gets larger in the direction that improves fit.

Worst-case is used when failure is unacceptable, and all contributors can align in the same direction. RSS is used when contributors are statistically independent, and the system can tolerate rare extremes. In interviews, the safe framing is worst-case for hard functional stacks and RSS for performance stacks, where controlled risk is acceptable.