The bonus tolerance is an additional tolerance that applies when the straightness callout includes a maximum material condition (MMC) modifier. In other words, as the actual size of the part deviates from the MMC size, the difference between the actual size and the MMC size is added to the straightness tolerance value. This additional tolerance is commonly referred to as bonus tolerance. For instance, let's consider a scenario where a pin needs to fit into a hole with a specified size at MMC. At MMC, the pin must have a high level of straightness or perfect form (according to Rule no. 1 of GD&T) to ensure proper assembly. However, as the pin diameter becomes smaller and deviates from MMC, the restrictions on the part's straightness become relaxed. In other words, as the pin size reduces to fit the hole, the part needs to be less and less straight. This means that pins with a waist, barrel, or concave/convex shape can still clear the designated go/no-go gauge designed for them as they deviate from MMC.