Certain file types such Xliff or PO (portable object) might contain additional information other than keys and their respective translation values. This information (meta-data) generally allows the source system to correctly map the keys back to it. If you import your keys into eclypse with one of these files, eclypse automatically recognizes this meta-data and saves it as a "Custom Attribute" for each key. This way, when you export your keys out of eclypse back into the same file format, eclypse can re-create a similar file that contains all the necessary meta-data for the source system. You can change this meta data or remove them if necessary. An example of custom attribute interface is shown below.

List of Autogenerated custom attributes

File FormatKeyDiscussion
Xliff (Unity)file_name, file_id, file_urlA file is the container for localization material extracted from an entire String Table. The file contains a reference to the original asset path and its GUID, so that Unity can import any changes to the file back into the same String Table.
Xliff (Unity)group_id, group_nameThe XLIFF standard uses Groups as a way to organize units into structured hierarchies. The Localization system treats each String Table Collection as a Group. It uses the Collection Name as the Group name, and the Shared Table Data asset GUID as the Group Id. This means that when the Localization system imports an XLIFF file, it can locate and update the original XLIFF Group without additional user input.
Xliff (Unity)key_idA Translation Unit represents an entry taken from a String Table. It contains the source text and the translated text (if it exists). The Translation Unit also contains a name and ID which maps to the Key name and Id, so that when the Localization system re-imports updated translations, it can correctly map them to their table entries.
PO file (Unreal Engine)unreal_engine_id, unreal_engine_namespaceThese attributes contain information about which collapse mode was used to generate the PO file from the Unreal Engine.
PO filemessage_context, source_code_ref,
Some of the comments in a PO file are stored as custom attributes with the keys described in this list.

See also

Unreal Engine - PO File Format

GNU Org - The Format of PO Files

Unity - Xliff Support