Shangish,
If we reference the release notes for UC 1.5:
https://pae-web.presonusmusic.com/downloads/products/pdf/StudioLiveAI_and_RM_Family_1.5_ReleaseAddendum.pdf
Stagebox Mode (i.e. “Simple” Stagebox)
This is a more traditional Stagebox function, where the RM mixer simply serves as a remote I/O box for the Console at FOH. All mixing, both FOH and Aux mixing, is done on the AI Console at FOH. NOTE: UC Surface/CS18 control of the RM mixer is NOT supported in this mode When using Stagebox mode, the mixer DSP on the RM is bypassed and all RM outputs are copies of the AI Console’s Auxes and Subgroup outputs as shown below: