JDXStudio, JDXDemo, and JDXSchema tools are explained in JDX Factsheet. For more details on these tools, check the JDX User Manual.
The TopDown steps are taken when the application development starts with a fresh object model. The database schema may or may not exist in this case.
The BottomUp steps are taken when a new application needs to be developed using an existing schema and the developer wants to get a jump-start by creating an object-model based on the existing schema.
JDX also supports the notion of meeting in the middle where both the object model and the database schema exist. In that case, just define the mapping and develop the application with JDX APIs. The reverse-engineering feature may be used to get a jump-start on the mapping definition; discarding the reverse-engineered class definitions as the object model (classes) already exists.