Settings: what does this icon mean?

Hello Becrylen, 

Thanks for contacting JetBrains support. 

The explanation of this icon is wrong and misleading. We're working to improve it, tracked under IJPL-40476 Settings: misleading hint "Settings marked with this icon are only applied to the current project". Please ignore it for now. 

The icons users should note are as below:

Rider uses two mechanisms to store its settings. The icons above indicate which mechanism is being used. Please see Rider settings | JetBrains Rider Documentation for more information. 

Thanks,

Tom