Why the path to the directory part in the Git URL is double slash //?

Terraform modules using a git branch as a source

git::ssh://private_server:myport/kbf/my_repository.git//ecs-cluster?ref=myBranch

git::https://private_server:myport/kbf/my_repository//ecs-cluster?ref=myBranch

Looking at GIT URLS page of the Git SCM but there is no // mentioned. Please advise where it is documented.