In IIS, default documents are files that are automatically served when a user accesses the customer's Web site but does not request a specific file. A default document might be the customer's home page or a file list (if directory browsing is enabled).
The default documents that are created in the Web site root directory are automatically passed to any subdirectories that are created.
Default documents can be modified at the root Web site level or at the subdirectory level. If a document is added at the root level, it is applied to all subdirectories.