The physical layer is the layer of the Logical Business Model where you define the physical data model of your data source. It contains information about the physical data sources.
The most common way to create the schema in the Physical layer is by importing metadata (table, view, …) from databases and other data sources. If you import metadata, many of the properties are configured automatically based on the information gathered during the import process. You can also define other attributes of the physical data source, such as join relationships, that might not exist in the data source metadata.
From 11g, importing physical objects from an Oracle Database can be done through a Oracle Call Interface (OCI)
table1.foreign_key_column = table2.primary_key_column
Catalogs are optional ways to group different schemas. A catalog contains all the schema (metadata) for a database object. A schema contains only the metadata information for a particular user or application. Model the physical layer after the way your database is structured.
A database can have either catalogs or schemas but not both.
If your database has one or more schemas, you cannot create a catalog. If your database has one or more catalogs, you cannot create schemas.
You must create a database object before you create a catalog object or a schema object.