Minecraft wiki entry for debug, associated text: [C] <command> means the <command> is executed.
[M] <message> means a message is returned.
[E] <message> means a failure message is returned.
[R = <num>] <command> means the <command> returns a brigadier return value.
[C] <command> -> <num> means the <command> is executed and returns a brigadier return value.
[F] <function> size=<num> means a function is called.