Final answer:
Clean outs of 2 inches or smaller require an 18" clearance in front of them to allow for proper maintenance and emergency access by plumbing professionals.
Step-by-step explanation:
The question pertains to plumbing standards regarding the installation of clean outs. These are access points in a plumbing system that allow for blockages to be removed and for the system to be cleaned. According to plumbing codes and standards, clean outs that are 2 inches or smaller in size are required to have a clearance of 18" in front of the clean out. This clearance is crucial to allow enough space for a plumber or technician to access the clean out for maintenance or during an emergency.