What Is ESS and MSS?

ESS - Employee Self Service is an web enabled tool used to update their own datas by the employee itself.

MSS- Manager Self Service is an web enabled tool used for Managers to take report or update the datas of subordinate.

This will give you a comprehensive idea:

To implement LEAVE REQUEST &APPROVAL, both ESS and MSS are not mandatory.
But ESS is required.However this functionality is developed to be avaialable in MSS also if a client wants have both ESS and MSS.  Without MSS, the business flow of Leave request would be in ESS  is like this:

1. An employee wants to take two weeks of leave. He or she accesses the Leave Request Web application. He or she sees in the Team Calendar that other team members also have leave at the same time. Nonetheless, he or she requests leave for the planned period.