Depth routing

You may be able to use Z-Axis but this is limited to MIn/Max, Relative Prop Delay and Diff Pair rules so you may need to add the distance on the starting layer into the calculation. If this doesn't help maybe some more information to clarify exactly what you are trying to achieve.

Hi

My question was related to drill away the white area as shown in the picture attached by 1.5mm.

That means after depth milling in the white areas, the thickness of PCB in that area reduces to 1.5mm while the remaining(Green) area around it is 3mm thick.

I don't think there's much in Allegro that can help you if a feature is not specified in the ODB++/IPC2581 standard.

You will need to specify this on a fabrication drawing.  Add an artwork layer with text that specifies what you are trying to request for the drawn polygon.

It will be very easy for your fabricator to miss this detail if you are not detailed/specific enough on the fab drawing.

I do this on my actual board by using a secondary stackup: just remove the unwanted layers in the area. Need to work at least in 17.2 because multiple stackup does not exist before.

It is designed by cadence for flex-rigid but works well in this case (rigid only).

Here is the result in 3Dview