It was their prisoner number.
blue white stripes
Almost all prisoners wore a uniform, much like in any other prison before or since.
Elie Wiesel kept a striped concentration camp uniform from his time at Auschwitz.
Striped pajamas, aka Auschwitz prisoner uniform.
Yes, they did! The uniform, being given the nickname pyjamas. In winter they would be given a coat of the same 'pyjama'-style.
Each of these striped uniforms was marked with a badge indicating the prisoner's category and an identification number that henceforth substituted his or her name.
Auschwitz I Stammlager, Auschwitz II Birkenau and Auschwitz III Monowitz
Symbols associated with St. Maximilian Kolbe include the interlocking M and A representing the Militia Immaculata, the Auschwitz prisoner uniform he wore, and a crown made of barbed wire signifying his martyrdom by offering his life for a fellow prisoner at Auschwitz.
Auschwitz Birkenau was established at Auschwitz but Auschwitz is now called Oświęcim.
Auschwitz I- Birkenau
It had 3 sections. Auschwitz-I, which served as a working camp. Auschwitz II-Birkenau, the death camp. Auschwitz-III, it was used to provide slave labor to the nearby industry.