$(tag) references the full tag name, $(tag) and $(tag) are the first and second values of log tag split on periods. You may access any member by index, 0 through 9. $(uuid) will insert a random string in the names. The random string is generated automatically with format: 4 bytes of time (seconds) + 16 random bytes. It is created when the plugin starts up and uniquely identifies the output - which means that until Fluent Bit is restarted, it will be the same. If you have multiple CloudWatch outputs, each one will get a unique UUID. If your container is running in ECS, $(variable) can be set as $(ecs_task_id), $(ecs_cluster) or $(ecs_task_arn). It will set ECS metadata into log_group_name or log_stream_name.