Roll-up summary sharing and field access
Key Takeaway
Apply field-level security to the roll-up summary field itself to keep its value away from certain users. Hiding the detail field does not hide the total.
A roll-up summary field shows a value on a master record in a master-detail relationship. It calculates that value from the master record's detail records, such as a count of those records or the sum of a field. The roll-up summary field lives on the master record but draws on detail fields. Its own field-level security therefore controls who can see the result, not the security on the detail fields it summarizes.
What a roll-up summary field does
A roll-up summary field can count the related detail records, or it can calculate the SUM, MIN, or MAX of a field on those records.
For example, a Total Invoice Amount field on Account can sum the amounts of the account's related Invoice records.
Roll-up summary fields are read-only on detail pages and don't appear on edit pages, so users can see the calculated value but can't enter one.