Merriam-Webster defines referral as "when someone sends someone or something to another person to be helped or dealt with" (Merriam-Webster, n.d., Definition 1).
This exactly happens when you encounter questions that you are not equipped to answer them and refer a library users to someone else. Use the reference interview to determine the library users' need and refer them to proper person or department.
Tips to a successful referral:
Research questions? Refer to the on call librarian.
Interlibrary loan, tutoring, book store, username and password questions? Refer to proper person/department.
Let the user know that you are referring him/her for more assistance. Ask them to wait while you contact the person/department receiving the referral.
Call the on call librarian Let her/him know you have a referral for her/him.
If the librarian does not answer follow the instruction.
Notes:
Do not send users away without help or a referral. If the subject librarian is not available, provide the patron with librarian's contact information and share subject guides to give the users additional resources to view until they meet with a librarian.