Within the header of a data load rules file, you can specify one member per dimension. For example, if you specify January, for all data-source records, January represents the Year dimension.
If a rules file contains a dynamic reference to data-source headers, the header information contained in the rules file is not used (because the header in the data source takes precedence).
To specify data-source members within rules file headers:
Select Options, then Data load settings.
In the Data Load Settings dialog box, select the Header Definition tab.
If the Dimension node is empty, click Outline, and associate the rules file with an outline.
If you enter a substitution variable, the variable value that is effective when data is loaded is used.
Click OK.
“Defining Header Records” in the Oracle Essbase Database Administrator's Guide