How can I secure the data in GL balances cubes?

Use data access set and segment value security to secure dimension values such as ledger and chart of account values.

For chart of accounts dimension values, security restricts the display of data associated with the secured values, but not the selection of the values themselves. For example, when submitting a report, you can select company value 100 in your report definition when selecting the Point of View, even if you weren't granted access to that company value. However, you can't see the data associated with company 100 in your report.