@RequiresRestart indicates that a marked field (or every field within a marked class) will need the client and/or server restarted in order for changes to take effect. If the user makes a change that requires a restart, they will be prompted in the chat menu, and a server log message will be printed. If a client joins to a server and the client finds that there are @RequiresRestart fields that don't match to the servers incoming config synchronization, they will be disconnected and prompted to either restart or go back to the main menu.

@RequiresRestart
public int myItemDurability = 250; // something like item durability would require a restart, as durabilities are immutable once an item is instantiated.

The screen show to clients if they joined with un-synced configs with differences that require a restart. Configs have been synced by this time, so when they restart and rejoin, the screen will not appear again.