Scenario:
How to restrict the sales reps from viewing other customer’s which are not assigned to them
Solution:
First identify whether the sales rep has custom role. Then login as administrator
1 Go to Setup>Users/Roles>Manage Roles
2 Edit the custom role
3 In Employee restrictions drop down field set Own and subordinates only . If you want them to see Unassigned entities select Own, subordinate and unassigned
4 Save the role