In GD&T, bonus tolerance is a modification of a GD&T tolerance that under certain conditions increases the tolerance, hence the term “bonus”. More specifically, when the maximum material condition (MMC) symbol is used to modify a GD&T tolerance, bonus tolerance becomes available. The MMC modifier implies that the GD&T tolerance can be increased if the manufacturing process can control the related feature-of-size.
An example of bonus tolerance is shown below. The top figure shows the drawing callout for the straightness of a cylindrical pin modified by an MMC symbol. The bottom drawing shows how controlling the feature-of-size can allow for more straightness tolerance. The logic here is if the feature-of-size is smaller than the MMC, than additional variation in the straightness should not prevent the successful assembly of this pin to its mating hole. For this reason, the manufacturing team may choose to intentionally make the part smaller in order to avoid the need to tightly control the straightness. Sometimes this leads to lower manufacturing costs since the part does not require special tools and equipment to control its geometry.