the simplest way is to use MEGA integrated help (press F1)
Then, in the Contents tab, expand the Metamodel > Metamodel > Object Types (Metaclass) folder, select the concerned metaclass, and in the main windows, read what you need about it :-)
I had the exact same question, but there is no more a complete description of the metamodel in the HOPEX V1R2 help. =)
Do he have another way?
Adding to Lionel suggestion, Below is another way to check this :
Explore the MetaClass then check the metamodel link and there you can see the main product when the parent metamodel is MEGA products