Ken,
Soft limits work from the tool zero point so so you have your zero located in the centre of the work which we will say is 100mm square but you can go 150mm in any direction before you run out of travel.

So set your work zero's at the centre, then go to Config > Homing and Limits and set the soft max in X and Y to 120 and min to minus 120, click OK and go back to the main screen and click on soft limits which is just below the DRO's to the right.

This now means it can only go 120mm in any direction before it stops on it's own.

One drawback with soft limits is that it's from work zero, move the origin point and you have to reset the limits.

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