Creating Logical Time Dimensions
Compared to modeling an ordinary dimension, the time dimension requires just two additional
steps: selecting the Time option in the Logical Dimension dialog, and designating a chronological
key for every level of every dimension hierarchy.
Note: Time dimensions and chronological keys. You can identify a dimension as a time dimension.
At least one level of a time dimension must have a chronological key. The following is a list of
some guidelines you should use when setting up and using time dimensions:
*At least one level of a time dimension must have a chronological key.
*All time series measures using the AGO, TODATE, and PERIODROLLING functions must be on
time levels. AGO, TODATE, and PERIODROLLING aggregates are created as derived logical
columns.
Reference:Oracle Fusion Middleware Metadata Repository Builder's Guide for Oracle Business
Intelligence Enterprise Edition. Working with Logical Dimensions

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.

Explanation
OBIEE - Grain (Level of summarization)
Grain definition in the context of OBIEE.
The following list describes the different grains in navigating a query:
* Aggregation grain. The grain of the aggregate source of from a level based metrics
* Query grain. The grain of the request.
* Time Series grain. The grain at which the time series function aggregation is requested.
* Storage grain. The query in the example can be computed from daily sales or from monthly sales, or from quarterly sales.
Note:
The aggregation grain is defined in the content tab of the logical table source.
When the query grain match the aggregation grain of a logical table source, this one is used to create the physical query.
http://gerardnico.com/wiki/_media/dat/obiee/bi_server/design/fact_table/obiee_fragmentation_content_one_colu

Screenshot below.