The caliper will be ready for measurements as soon as it is switched on with the LCD screen displaying the actual slider position. There is no need to set the zero position each time the caliper is used as the origin setting is retained even when the caliper is switched off.
The ZERO/ABS changes the measurement mode from absolute to incremental. This is useful if a temporary origin is required to set the nominal value as zero for repetitive measuring task. By pressing the Zero/ABS button the display value will change to the original zero point (usually with the jaws in a closed position) and the measurement mode will be displayed.