Coat pattern in cats is set by the distribution of pigment-producing cells and by signalling molecules that switch hair follicles between producing dark and pale pigment. In tigers this patterning is expressed in the skin itself, not only in the hairs growing out of it, so the stripe map remains visible on shaved or bare skin.
Because the pattern is fixed and asymmetric, it functions as an identifier. Camera-trap surveys across India and the Russian Far East match individuals by their flank stripes, which is how national tiger estimates are assembled without capturing animals. The left and right flanks differ, so surveys generally need paired cameras to avoid double-counting.
A common overstatement is that stripes are "like fingerprints" in the forensic sense of guaranteed uniqueness. What is established is that patterns are individually distinctive enough for reliable matching in practice, particularly with software-assisted comparison; that is a statistical claim about identification, not a proof that no two tigers could ever share a pattern.