You can create access rules to prevent specific users from editing specific plan cells. This can be useful when you close a time period, if you want to prevent users other departments from editing actuals, and so on.
When setting up an 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 plan cells are affected by the access rule, making them read-only.
Access rules restrict existing access, they do not provide additional access. Therefore, if a user has read-only access to a plan, the access rules for that plan do not affect them. Similarly, if a user doesn't have access to a plan, the access rules for that plan do not affect them. If you want to restrict a user's access to an entire plan, use the Salesforce sharing feature and permission sets instead of access rules.