It works like this: A lab somewhere in the globe wants to make sure their instruments are working properly. They buy a reference material, like the peanut butter, and break it down. The readings of those devices are measured against the standards set by NIST.
The agency constantly checks for the products they provide. 2387 is popular because fatty foods are highly tested.
Gizmodo writes: “The strangest part of the SRM catalog may be the array of human products: hair, urine, lung powder, liver powder, and a whole lot of serum. The products are also sorted by what they contain: THC, vitamin D, arsenic, etc.”