If you don't choose a content databases, all available content databases will be used. If you choose more than one content database, the workspace will be created in the content database which has the highest free capacity. Also the content database status is included in the prioritisation. The expression can return one or more names of content databases. You can also use the asterisk operator to provide 'begins with' names (e.g. 'MyContentDBNumber*'). User input can be accessed by a placeholder using the 'Self' variable, e.g. '{Self.FieldName}'. See 'SiteId' (on the node 'WorkspaceTemplateConfiguration') for more information.