The question is not cristal clear
If your question is 'Is there a reference documentation for SQL tables or SQL queries used in HOPEX with SQL Server/Oracle storage?' the answer is no. MEGA considers this as an internal implementation and it is not documented.
If your question is 'Is there a reference documentation for concepts (MetaClass) used in HOPEX?' the answer is yes.
However this not a document. MEGA Studio enables to browe the metamodel using query tool, explorer tool, property page.
Metamodel diagram can also be used to understand specific areay of the metamodel
ERQL queries are not documented. You can understand the metamodel browsing using the explorer tool.
Experienced users understand their meaning by reading the ERQL code.
What I would do if I was you is to use the MEGA API to export the needed data of the Application and the diagrams and store them to different database.
What do you mean by "register the database schema of applications registered in MEGA"?
Can you provide more details on what you intend to get?