How 3Dmax divides the object into two planes

First use the slicing command (convert to editable polygon if the command is not available) and

select the sub-level to change the plane position.

After slicing, a split surface will be formed. Here, select [Split Grid], and then you can split two elements.

Then convert to editable polygon, select the element level and

then separate the elements.
The separated elements are not sealed, select the boundary level

and then seal