Something is being excluded from being regulated by a specific law because it is being regulated elsewhere by a different law. For example, nuclear wastes are excluded from the Resource Conservation and Recovery Act (RCRA) because they are regulated by the Nuclear regulatory Commission. Domestic sewage is excluded from RCRA because it si covered under the Clean Water Act.
Exemption is a bit different. Something is exempt from being regulated or it is being less regulated because it meets certain conditions. For example, used oil is exempt from being regulated FULLY as a hazardous waste if it is being recycled. Universal wastes is another example. they are basically hazardous wastes but they are exempt from the full force of RCRA if they are recycled.