External identifiers enable you to assign custom identifiers to your assets so they are easier to organize, update, and modify. They are sometimes used to correlate with your own internal identifiers.
An external ID can contain any characters except commas (",") or slashes ("/").
The same external ID cannot be assigned to two different assets.
When making a change to an asset, you can use either its content ID (asset ID or embed_code) or external ID.