Note 1: There are four types of variables that you can use:
Session
Repository
Presentation
Request
Note 2: Session Variables
A session variable is a variable that is initialized at login time for each user. When a user begins a session, the Oracle BI Server creates a new instance of a session variable and initializes it.
There are as many instances of a session variable as there are active sessions on the Oracle BI Server. Each instance of a session variable could be initialized to a different value.
There are two types of session variables:
System - A session variable that the Oracle BI Server and Oracle BI Presentation Services use for specific purposes.
System session variables have reserved names that cannot be used for other kinds of variables (such as static or dynamic repository variables and non-system session variables).
Non-system - A system variable that the administrator creates and names. For example, the administrator might create a SalesRegion non-system variable that initializes the name of a user's sales region.
The administrator creates non-system session variables using the Oracle BI Administration Tool.

Identity Store: It is directory server to perform the authentication. It contains user name , password and groups membership information .When a user name and password combination is entered at log in, the authentication provider searches the identity store to verify the credentials provided.
See figure below.

Pull the table and column definition into the physical layer
Use the OBIEE "import metadata" wizard to pull the table and column definition into the physical layer. The wizard is used so that all of the column definitions do not need to be manually added into the physical layer.
Figure:

Note: 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.

A: OBIEE Federated Repository
Instead of moving data in to a single database, OBIEE - BI Administration can access multiple databases and create a single federated repository.OBIEE BI Answer uses this federated repository and build the ad-hoc reports and dashboards.
D: Repository Metadata Consists of 3 Layers
* The Physical Layer (Schema)
* The Business Model and Mapping Layer
* The Presentation Layer
Note:

Note: The Administration tool is used to manage OBIEE server and have some capabilities to automate some repetitive tasks and principally his embedded tools.
Oracle Business Intelligence Administration Tool is available only for Windows operating systems.
The Administration Tool is a Windows application that allows the BI Server administrator to create and edit repositories.
Repositories can be edited with the administration tool in either online or offline mode.
The Administration Tool also includes a session manager and a cache manager which are available in online mode. The Cache Manager displays what queries have been cached and how frequently they have been accessed. You can also select and delete items