Interface calculations

What do these regions represent in the interface calculations given in gulp example ?
cartesian region 1
cart region 2
cartesian region 3 rigid z

Also why there are 2 Mg cores without a shell , in the example given ?

They’re explained in the manual & help text for the commands (as well as the basic papers that introduce how surface calculations/core-shell models work). If you don’t want to read the documentation, then the person who is leading the research design of your work should be able to explain this to you.