I have little bit confusion on user right for security access and key roles and functionality access for Human Resource concepts in Odoo.

I have too much confusion around the HR Officer Users and HR Manager Users from the Whole Human Resource Concepts in Odoo.

I have found in some case from HR manager Having the full access not more then the Administrator but less then access to the Officer Users In Human Resource Management .

Can any one help me how both the users and its key roles played in whole Human Resources in Odoo ? and also

What is reason behind for add that two users into our Human resource flow Odoo ?