settings – What is the “phone index” in the hidden menu for “phone info”?

Device: OnePlus 7 Pro (GM1917), Oxygen OS 10.3.8.GM21AA, stock ROM

When I enter the hidden menu *#*#4636#*#* and select “Phone info”, the first line under “Phone info” says “Select phone index” with a drop-down menu:

All of the info listed after “Select phone index” is the exact same no matter which index I choose: Phone 0 or Phone 1.

What is a phone index? Why are they identical?