Although it is similar to stigmata it has one very essential difference.
Stigmata lasts for years or an entire lifetime.
Skin writing lasts only for a few minutes usually. In fact fraud in skin writing is widespread. They then rub their forehead or arm with the ashes which gives the opportunity for covertly tracing the message. Skin markings have also been produced by mere suggestion in experiments with hypnotism. Sometimes the skin writing is pictorial.