The --detach flag, when used with the docker service create or docker stack deploy command in Docker Swarm, runs the service in the background. It means the command will return immediately, and you can continue to use the terminal for other tasks without being attached to the ongoing deployment process.

For example:

docker service create --detach --name myservice myimage

This flag is useful in scenarios where you want to initiate the service deployment and let it run in the background, allowing you to perform other tasks or exit the terminal session without affecting the ongoing service.

Without --detach, the terminal remains attached to the service logs, and you see real-time output. When --detach is used, you can check the service logs later using the docker service logs command.

docker service logs myservice

This flag is particularly handy in automated deployment scripts or scenarios where you don't want the terminal to be tied up during the entire deployment process.