DotNetNuke Skinning Whitepaper (File Organization section)

File organization skins can be applied to the host, site, or paging level. The skin can also be applied to the module level, but we call it a container. All skin files are saved under the Portals folder. There is a special folder named _default that represents the host's folder, however each site is together in the database with the ID name of the respective folder. Such a structure allows the entire Web site to have access to read and write folders in a single folder. At the same time, the physical organization of the file is linked to the logical application within the application, so that the relationship between the two can be managed without the need for external database support.

Skins and containers can contain an unlimited number of subfolders-each folder represents a package that is necessary to make up the skin. Subfolders are saved with the name of the skin. Using a name increases the likelihood of duplicate names between different skins, but this eliminates the need to keep in line with external data sources when you manage files directly through the file system.