There are 3 permission related to Customizable Forecasting in Profile, they are located in General User Permissions.
Allows users to override their own forecasts and forecasts for users below them in the forecast hierarchy. All standard profiles except Read Only receive this permission.
Edit Personal Quota
Allows users to change their individual quotas. All standard profiles except Read Only receive this permission.
Users with the “Manage Users” permission can always edit any quota. Users can always edit the quotas of users that report directly to them.
View All Forecasts
Allows users to view any forecast regardless of their place in the forecast hierarchy. The System Administrator profile includes this permission.
As system admin, we need to make sure above permissions are given correctly. Otherwise, it may cause big issue.
Scenario: my user create a dynamic dashboard from Customizable Forecasting: Forecast Summary report type. When dashboard is viewing as a Country Manager, it suppose to show only forecast and closed amount from Sales Rep in the country based on Role hierarchy. But in this scenario, the Country Manager can see ALL Sales Rep from ALL country, which is not correct.
Solution: found it caused by the Country Manager profile have View All Forecasts permission granted.
A trace back, we give Country Manager profile this permission because there is a dashboard view as Region Manager and not as Forecast Manager. This user is assigned to the same profile but in the higher role hierarchy. After change the Forecast Manager to Region Manager and remove View All Forecasts, all working good.
Note: you can only have one Forecast Manager in a Forecasts Hierarchy. To setup Forecasts Hierarchy, go to: Setup - Customize - Forecasts (Customizable)- Forecasts Hierarchy
Key takeaway: Users with the “View All Forecasts” permission can see all forecast data of any user in your organization. It is recommended that you do not give this access to all users.