Maximum Material Condition, or MMC for short, is a feature of size symbol that designates a feature or component's state when the most material (in terms of volume or size) is present within the component's dimensional tolerance. The callout also nullifies GD&T, which asserts that feature size has no bearing on how geometry tolerances are managed.
Max Material Condition is one of the dimensional limits on a part. The other side of the tolerance range would be the Least Material Condition.
The only GD&T Symbols where Max Material Condition can be applied are:
MMC can be used to either guarantee that two parts never interact with one another or to reduce the amount of interference between the parts when their tolerances are at their tightest. Consider a shaft that needs to pass through a gap-free hole.
The MMC of the shaft would be the Maximum diameter.
The MMC of the hole would be its Minimum diameter.
There will always be clearance between the parts if you make sure that the MMC of the shaft is always less than the MMC of the hole. This is crucial for any tolerance stack to guarantee that the component continues to work properly even when the tolerances are in their least ideal state.