Assuming you have a typical setup with three RTPs, so left one will be VHF1, right one VHF2 and somewhere else there will be VHF3 - that is the normal setting for each of those panels.
If you now select VHF2 on the left RTP, the offside tuning light will illuminate on both the left and the right RTP, signaling:
- On the left RTP: that it is used for VHF2 instead of VHF1
- On the right RTP: that VHF2 is now also selected on the left RTP
Generally you should avoid using RTPs in a way that brings up the offside tuning light, except if you need HF or you have a failed panel/radio.