How can I ensure a person has access only to a specific segments and not everything?

Question 

How can I ensure a person has access only to a specific segments and not everything?

Answer

You will want to ensure they are added to a 'managed employee' access control group which dictates anyone in this group will only have access to their reporting line or any specific segments they have access to. The 'all employees' access control groups will give a manager access to company wide data, which you want to avoid if you need a user to have access only to a certain group of employees.