Some baggage restrictions are set by the U.S. government's Transportation Security Administration and apply to all airlines. There may be additional restrictions depending on the airline.
The TSA has a set of baggage restrictions all airlines must follow under penalty of legal action. These include a ban on firearms and other weapons or limits on transporting liquids. However, each individual airline is allowed to have additional restrictions, such as limiting the number of items customers may bring with them. For example, American Airlines limits travelers to five bags when flying to the Caribbean, Central America or South America and to 10 bags when flying domestically or to most other countries.