Important information describing an ArcGIS item may exist in different files such as reports detailing how the data was collected or about quality tests performed on the data. You might also have graphic files illustrating where an item is located or where the data was collected. While this information can be documented within the item's metadata or the file can be referenced from within the item's metadata, if it is important or useful enough to keep a copy of the original file with the ArcGIS item, you can enclose a copy of it within the metadata.
Enclosing files in an item's metadata works the same way as enclosing files in an e-mail message. The enclosed file is a copy of the original. If the original file is later modified, the copy stored within the metadata won't change. The information in an enclosure isn't available when you search for items. For enclosed images, you will see a thumbnail of the image when you view metadata.
Because metadata is part of an ArcGIS item, the enclosure also becomes part of the item. If you share the ArcGIS item with someone else, you are also sharing the enclosed file. However, when you import metadata from another ArcGIS item, any enclosures in the source metadata won't be imported to the target ArcGIS item.
Thumbnails are stored within an item's metadata in the same manner as enclosures, but are not listed in the Metadata Properties dialog box with the other enclosures. Thumbnails are managed separately. However, if you have published metadata to an ArcIMS Metadata Service and you look at the published document's enclosures, its thumbnail will be listed as an enclosure in the Metadata Properties dialog box. This is also true of metadata documents that were retrieved or exported from an ArcIMS Metadata Service. Do not delete this enclosure, or the thumbnail will be removed.
Enclosed files have a significant impact on the size of the metadata. If you find that you are attaching a copy of the same file to many items, the file should be referenced from the metadata rather than being enclosed within it.