Where can we found full description of “Xojo Database Server License” and of the other products?
Link please.
From Xojo FAQ.
"
With a Database Server license, your apps can connect to Oracle, Postgres, ODBC, Microsoft SQL Server, and MySQL. For a small number of users, you can use SQLite in its multiuser mode (for use with web apps). A Database Server license is not required to connect to SQLite."
Basically you need it to connect to other databases than SQlite 
Full description, please. Where is it? Nowhere.
If you are using the provided database APIs. There are other database APIs that do not require this licence. You do, however, need to buy the alternatives. For example MBS provides it’s own database plug-ins using a different API.
Not currently available. It’s very new.
It’s ok. I’ll be waiting.