Height Gauges

I just got a LCD Height Gauge (iGaging) for Christmas.
I have no problem using it. It can be reset to zero at any one time.
One of the function key is ABS: absolute measuring and incremental.
Press ABS key to get relative zero. I am not familiar with this ABS.
Especially when I can press the Zero key setting at any position.
Maybe some of the readers have one already and can tell me
what ABS is use for? TIA
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