Combat Tag is a mechanic which prevents a player from logging out during combat without consequences. When a player damages or takes damage from another player, they are marked as combat tagged for some period of time. Any player that logs out while tagged will leave a vulnerable body behind for a period of time that can be attacked and, in particular, pearled.
Mechanics
Whenever a player takes damages or takes damage from another player, they are marked as combat tagged for a period of time. If a player logs out while tagged, their body will remain on the server for a period of time. They can still take damage, be killed, and be pearled while in this state, just as if they were logged in.
When a player logs into the server, they are also usually automatically combat tagged for a short time.
Players can log out safely (without leaving their body behind) with the /logout command. This command requires that the player wait a short period without moving or taking any other action.