Horizontal Integration Facility

A horizontal integration facility (HIF) is a location within which the stages of a multistage rocket are brought together, before the assembled stack is rolled out to the launch pad or complex (LC) and raised into vertical position for launch.

Most Russian, Ukrainian and former Soviet launchers are integrated horizontally, including Dnepr, Proton, Rockot, Shtil' and Soyuz.

...
Wikipedia