The words "chosen for processing" are a euphemism for death in the concentration camp. In chapter 15, Hannah hears that there is going to be a "Choosing" in the camp. Not understanding the term, Hannah asks Rivka what this means. She explains that anyone who could not get out of bed that morning will be chosen for processing. In fact, anyone who is unable to work will be chosen.
Hannah understands that this means that they will be chosen for death. Rivka urgently tells Hannah never to speak the word death. They cannot even say corpses or killed. Hannah explains that the Nazis do not want the prisoners speaking of death. Leye further explains that they speak this way "because what is not recorded cannot be blamed."
There is a sick irony to calling the condemned Jews "chosen." The Jewish people often refer to themselves as the chosen people of God. By calling them chosen in this context, the Nazis are both mocking their Jewish prisoners and undermining their religious beliefs. Also, the use of this euphemism underscores the cognitive dissonance of the Nazis at the camp. They treat the prisoners as less than human and murder them without remorse. Yet, they still feel that it is necessary to hide the reality of their crimes by not calling them what it truly are.