Practicing the art of deception is considerably important when there is no open gate available for an auditor to enter the target network. Thus, using a human attack vector, it is still possible to penetrate the target system by tricking a user into executing malicious code that should give backdoor access to the auditor. Social engineering comes in different forms. This can be anybody pretending to be a network administrator over the phone forcing you to reveal your account information or an e-mail phishing scam that can hijack your bank account details. Someone imitating personnel to get into a physical location is also considered social engineering.
There is an immense set of possibilities that could be applied to achieve the required goal. Note that for a successful penetration, additional time to understand human psychology may be required before applying any suitable deception against the target. It is also important to fully understand the associated laws of your country with regards to social engineering prior to attempting this phase.