/callback is an internal sponge command. When a plugin sends a message to the user, the plugin can define a function that will be run when some text is clicked. However, the client only supports running commands. To allow plugins to directly run a function instead, sponge provides a command /callback that when given the id of a function, will run that function. This is a command that is present on every Sponge server and which is harmless to allow since it can only function when a plugin specifically requests it.
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