Within the Red Hat certification program, the RHCA is considered the most complete (and grueling) certification among the seven major Red Hat certifications. In fact, some equate the prestige of the RHCA to that of the Microsoft Certified Master and possibly the CCIE.
A plumbing fixture like a sink can not function properly without a sink vent. When a drain pipe drains it creates a negative pressure in the sink drain; the negative pressure will pull the water out of the p-trap of the sink that's draining and sewer gases can enter your home if a sink vent is not installed.
When does an under the sink P-Trap assembly need venting? In the realm of second homeowner woes, I had a P-Trap assembly under the bathroom sink. It was a strait connection to the pipe connected to the main sewer line. (least I think that was the case) either way it was not vented and it worked just fine.