Others have already answered this but l'll add my voice. These are "controls" for their test. Standard practice is to have control samples ship and be stored with ypur actual samples to test for contamination. These conteol samples have a known result. If the control samples don't display this known result when tested, you know something is bad with your sample batch. An example is if a sample batch gets too warm or sits for too many days and gets degraded. Control samples are used in every sampling scheme I've ever seen (I've done environmental sampling).