There seems to be a misconception here. Auschwitz was a combination death camp and work camp. This meant that in addition to having gas chambers and crematoria for mass executions, it also kept a large body of prisoners for slave labor to work in places such as I. G. Farben's synthetic rubber plant. The selection process for Jews was where camp officials would decide who would be fit for such slave labor and those who were not. Those who were deemed unfit went to the gas chambers. The criterion for such a selection was neither very sophisticated nor meticulous. Children and the elderly were typically among those automatically selected for gassing, but this was not always the case. Elie Wiesel, for example, was still a young teenager when he was selected for work. Survivor accounts such as those of Primo Levi's also speak of a very capricious selection process in which camp officials never had any real consistency to their decisions.

Tattoos and other forms of identification were for those prisoners who were not targeted for immediate extermination. This included non-Jews who were inmates such as Soviet PoWs as well as those Jews who survived the selection process. These serial numbers helped identify prisoners and became ubiquitous among survivors of Auschwitz. The tattoos were a way to keep track of the prisoner population as it both grew and less invasive forms of identification such as numbers sewn into prison uniforms became impractical. The growth of Auschwitz's inmate population, fed by the extermination process, meant that earlier methods of tracking just could not keep up with increasing number of bodies being fed into the camp system. The high mortality due to forced labor and scarcity of material goods also meant prisoners would often scavenge the dead and dying of their prison clothes. The tattoos also promised to keep some of the racial classifications intact as the population became more heterogeneous. There was a system that classified prisoners by racial hierarchies; this indelible marking in theory would prevent Jews and other racial undesirables from blending into other elements of the prison population and evade their eventual destruction.