Section 4, pg. 25 of the CS18 Owner's manual:
The CS18AI is primarily a control surface, but its rear panel includes a selection of audio inputs and outputs. These are convenient for providing talkback and for playing source audio from front-of-house and from the CS18AI headphone/monitor outputs.
By default, CS18AI audio is routed as follows with a StudioLive RM mixer:
Mic/Line input 1/TB is assigned as the default source for the StudioLive RM mixer Talkback. It will also be available as the network input source for Channel 31 of the RM mixer.
Mic/Line input 2 is unassigned by default but will be available as the network input source for Channel 32 of the StudioLive RM.
Line inputs 3 and 4 are assigned as the network source for the stereo Digital Return input of the RM mixer.
The inputs on the CS18 don't provide additional channels to the RM, they only serve as an alternate source for existing channels on the RM (31, 32, Dig Ret L and R).
The Line inputs on the CS18 do not have adjustable gain. Only the Mic Preamps on the CS18 Inputs 1 and 2 have adjustable input gain (accessible from the Talkback Edit screen, per pg. 50 of the Owner's Manual).
The Digital Return channel does not have Fat Channel EQ capability, but Ch. 31 and 32 do.