Follow-up: Why tattoo prisoners instead of branding them? Wouldn't branding have been quicker and have required less skill? I mean it's not like the Nazis cared much about being humane to their prisoners.

The United States Holocaust Memorial Museum has details on tattooing. Particularly relevant passage: "Originally, a special metal stamp, holding interchangeable numbers made up of needles approximately one centimeter long was used. This allowed the whole serial number to be punched at one blow onto the prisoner's left upper chest. Ink was then rubbed into the bleeding wound.

When the metal stamp method proved impractical, a single-needle device was introduced, which pierced the outlines of the serial-number digits onto the skin. The site of the tattoo was changed to the outer side of the left forearm. However, prisoners from several transports in 1943 had their numbers tattooed on the inner side of their left upper forearms. Tattooing was generally performed during registration when each prisoner was assigned a camp serial number. Since prisoners sent directly to the gas chambers were never issued numbers, they were never tattooed." They did not bring tattoo experts in to tattoo because they only cared about identification (and potentially humiliation/degradation), not about creating a tattoo that looked good. I don't think they provided any particular training in relation to tattooing. (I have certainly never encountered any references to this in books about camps.) Why waste resources on experts?