Rem Dim (or Remainder Dim) is a feature common on most lighting desks which dims all active outputs to 0%, and sends all channels you have selected to 100%. This feature is particularly useful during focusing sessions or rig checks.
E.G. Dimmers 1 thru 12 are at 50%. You want to specifically pick out dimmer 6 to allow you to focus it better. Select channel 6 and 'Rem Dim', channels 1 thru 5, and 7 thru 12 go to 0% and channel 6 goes to 100%. In ZerOS, channels affected by Rem Dim are shown in Yellow as @kgallen has pointed out earlier.
I don't believe Rem Dim is a feature that is enabled on the FLX S range consoles, and is exclusive to full FLX only (unaware of other legacy consoles). On review of the ZerOS Shortcuts (https://www.zero88.com/manuals/zeros/shortcuts/keyboard-shortcuts?filter=1) "Shift + F" enables Rem Dim on FLX but I have tried this on Phantom and it doesn't work for FLX S. This may be an error that this feature works via an Apple Watch (unable to test this unfortunately!)
You do have access to the 'Highlight' feature (Shift + Home) on FLX S range. Highlight allows you to temporarily bump the intensity of a selected fixture up to 100%, which again can also be useful during focusing.
During testing on Phantom ZerOS, both Rem Dim and Highlight can be 'CLEAR CLEAR'-ed.
Hope this is of some help! Good luck!