A cell access rule limits which forecast cells specific users can edit. This can be useful when you close a time period, if you want to prevent project managers from making adjustments to actuals, and so on.
When setting up a cell access rule, you can define the following:
The dimension values the rule applies to.
The measures the rule applies to.
The users the rule applies to.
The combination of those three elements determines which forecast cells are affected by the access rule, making them read-only.
Cell access rules restrict existing access. They do not provide additional access. Therefore, if a user has read-only access to a forecast, the cell access rules for that forecast do not affect them. Similarly, if a user doesn't have access to a forecast, the cell access rules for that forecast do not affect them. If you want to restrict a user's access to an entire forecast, use the Salesforce sharing feature and permission sets instead of access rules.