A backflow prevention assembly protects the public drinking-water system from a reversal of flow at a customer connection. SFPUC lists boilers, chemical-feed equipment, dedicated irrigation, interconnected fire sprinklers, taller buildings, and elevated water supplies among the property conditions that can create a potential cross-connection or hazard under its ordinance.
The annual field test is the operating evidence that the required assembly still performs. A tag, old report, invoice, or photo does not establish the result of the current test. If an assembly fails, the compliance workflow normally continues through repair or replacement, a passing retest, and the report or portal submission required by the serving utility.
Utilities therefore have a water-quality reason to enforce the program. The exact enforcement tool is local, but the underlying control is consistent: identify the hazard, use the required protection, test it on schedule, and document the result.