This apply only for repository creation so only POST route is affected:

POST /svn

{
    ...
    "settings": {
        "commit_rules": {
            "mandatory_reference": boolean,
            "allow_commit_message_change": boolean
        },
        "access_file": [
            "[/]",
            "* = ",
            "Contributors = r",
            "[/stuff]",
            "Contributors = rw"
        ],
        "immutable_tags": {
            "paths": [
                "/tags"
            ],
            "whitelist": [
                "/tags/moduleX"
            ]
        },
        "email_notifications": {
            [
                {
                    "path": "/trunk",
                    "user_groups": [
                        {
                            "id": "101_3"
                        },
                        UserGroupRepresentation
                    ],
                    "users": [
                        {
                            "id": 333
                        },
                        UserRepresentation
                    ],
                    "emails": [
                        "foo@example.com",
                        string
                    ]
                }
            ]
        },
        "layout": [
            "/trunk",
            "/tags",
            "/branches",
            "/branches/moduleX"
        ]
    }
}

• layout will create listed directories as "Revision n°1" of the repository
• Only folders are created, it's not possible to create files
• If the given layout cannot be created there is no error returned but the corresponding system event will be marked as "Warning"
• The layout is created in 1 commit, this means that it's "all or nothing", if something prevent the the creation of one path or the whole commit it self, nothing will be created
• The commit is author is the user who made the REST call.