The usable specimen thickness for each family depends on the material as much as the instrument: flash needs the rear-face rise slow compared with pulse and detector but fast compared with heat losses, so its window slides upward as diffusivity rises. Drag the material from insulating polymer to metal and watch the laser-flash band slide to the right — that movement is real physics, not a glitch: a fast-conducting material finishes its rear-face rise too quickly in a thin disc, so it needs a thicker specimen to stay inside the method’s valid timing window. The other bands are set mostly by the apparatus, so they stay put.
Bands are representative teaching windows illustrating how validity limits move with material diffusivity; every instrument publishes its own validated range and competent labs extend them with corrections.