delegate user Servicenow

Hi Everyone

I need Know About Delegates in User Table  I have Three Users  User A Have Admin Access, User B Have Itil role User C Have Snc_role 

Suppose User B Going to Leave  he suppose to delegates Has his work to User C   He Has Only Snc_internal  role  

In this Case User B Role delegates to User B Other Wise We need to Give the role to user C 

Please Give me Brief On Delegates  

My Quation is User B Have Itil User  Have Snc_internal  User B Delegates his work to User C How can he Achieve 

Is There Any OOB functionality available