An ear tag usually carries an Animal Identification Number or code for the animal, or for its herd or flock. This identification number may be assigned by some national organisations or they may be handwritten for the convenience of the farmer ("management tags"). Each producer uses his own numbering system.

However, if animals are tagged for internal purposes in a herd or farm, IDs need not be unique in larger scales. Cattle are not the only animals identified by tags or other methods. Any herd or flock of animals raised for profit is marked similarly. Pigs, chickens, sheep, goats and rabbits are a few examples. Animals are also tagged for research purposes, such as flocks of geese to determine migration patterns, and endangered species to prevent extinction. Ear tags are used by the livestock producer to keep production records of bloodlines, birth dates, vaccinations and other key criteria necessary for raising animals as a career.

Without some form of identification, it would be impossible to manage the records of each animal, especially when entire herds are raised by one person or family. An ear tag can be applied with an ear tag applicator, however there are also specially-designed tags that can be applied by hand. Depending on the purpose of the tagging, an animal may be tagged on one ear or both. If there exists a national animal identification programme in a country, animals may be tagged on both ears for the sake of increased security and effectiveness, or as a legal requirement.

If animals are tagged for private purposes, usually one ear is tagged.