Content Identifiers/Content Tags
Location - Utilities
This feature is used to assign identifier tags to a certain content type. A common use of this would be for a site that has various videos at difrerent aspect ratios and resolutions that must be detected by the system before or after being encoded in order to have them sorted and displayed properly on a site. This is not a feature that is likely to be used by a client, it's primarily used in custom site setups.
Note - In versions prior to 3.1 this is labeled "Content Tags" on the Utilities Page.