In some standard surround formats for cinema - often denoted as n.1 surround the .1 refers to a dedicated channel for extra sub bass effects in cinema. The LFE channel is normally sent directly to the sub-woofer speakers in a real room.
SPAT dœs not model sub-woofers or spatialise their acoustic information in the Virtual Rooms. But it dœs handle the channel in a special way. When working in a purely Channel Based signal graph, with Channel Based rooms outputting to an n.1 surround speaker arrangement, each source will have an individual LFE Send which controls how much of its signal is sent to the LFE channel at the output.
This parameter is available inside the Room editor, but also available directly from a Source module in the Environment Setup graph.