It is complex to explain in details how this parameter is considered, but basically the toolpath is generated twice- once considering it (1) and second time ignoring it (2) and then paths are compared. if a cut in path (2) do not exist in path (1) it means that it should be ignored. if a cut in path (2) exists in path (1) (partially or entierly) it means that it should remain.
The previous operations you defined - not only leavse material based on their stock (1mm, 0.4mm) but also based on the tool size and shape and the depth of cut will leave stairs of material on slanted walls.
the question you have to answer is if you want the rest cavity to machine the entire part as a semi finish operation (then you can set MMR to 0), remove the stairs left from Z level (then set it to 0.4 or slightly more) or only rough places where the previous tools could not get in (then use a larger value dependent on the material left after Z-level in the areas the Z-level operation could machine).
There is a top value you should not pass and that is the depth of cut you are going to use in the rest cavity operation. the reason is that if you exceed it the system may remove a complete cut level in some cases cuasing to a "heavy cut" at the begining.
sorry for the length and complexity of the expalnation..
As I wrote above - if you want all area to be machined with the new operation to reduce the stock from 0.4 to 0.12 (like a semi finish) - set it to 0.
if you want it only to machine areas where previous tool cannot get in (like corners, channels etc) - set it to be slightly smaller than depth of cut.
You can try with both settings and see which better fits your need.