MXXXCore is an modular way to get MXXX, the ultimate effect. It's cheap and you can buy only those modules you actually want.
MXXXCore is a special licence, which includes only the basic building blocks. You will use MXXX as if you had the full licence, but not all processors will be available.
It does of course includes all of the modules that MXXX does, but you will need a licence to activate them. For example, if you own MSpectralDynamics (or any bundle which contains that plugin) then it will be enabled in MXXXCore as well. MXXXCore will not allow you to insert a processor that you don't own nor open a preset that uses that processor, and it will tell you what you would need to purchase in order to get it.