Use for all matters related to water-based, fixed fire suppression systems (often called "fire sprinklers").
Fire sprinklers are most commonly found in multi-family and mixed-use buildings as well as their more traditional domains of high-rise and commercial/industrial construction, but there are also jurisdictions where they can be found, or are even required, in single-family homes and duplexes.
This tag covers all residential fire sprinkler applications -- non-residential applications often involve more complications than can be reasonably dealt with in the DIY context, and should be discussed on Engineering.SE instead.
Note that the design & installation of the actual sprinkler system generally is considered a professional-only project by local codes; even if that is not the case, it requires a good working knowledge of NFPA 13/13R/13D i.e. the generally adopted North American fire sprinkler code, or the local equivalent for those outside North America. However, use this tag when a sprinkler system is present in the vicinity of a project that involves other building systems, as NFPA 13(R/D) contains special requirements in it that must be followed to keep other building systems from interfering with the sprinkler system's operation.