J-Box
A J-box, or junction box, is an enclosure that houses electrical connections, protecting the joins between cables and keeping them accessible for inspection.
Wherever cables are joined, the connection must be enclosed. The junction box contains any fault, prevents accidental contact, and keeps the connection reachable, which is a requirement of the wiring rules.
In residential construction, junction boxes commonly sit in ceiling spaces, wall cavities and subfloors. They must remain accessible, so they cannot simply be sealed behind a lining with no way back to them.
Coordinating junction boxes and other services during documentation avoids the familiar problem of a fitting landing exactly where a structural member or a joinery unit needs to be.