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 subwoofer speakers in a real room.
SPAT does not model subwoofers or spatialize their acoustic information in the Virtual Rooms. But it does handle the channel in a special way. When working in a purely Channel Based signal graph, with Channel Based rooms outputting to a 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.