Definitely second Ben's suggestion to read the Admin Guide, but here is a quick explaination of ADAMM
http://seer.entsupport.symantec.com/docs/192265.htm
Basically, if you want to run a backup every night, and keep seven copies, set the OPP of the media set to 6 days (if the job finishes after 2400 or 7 days if it finishes before 2400), and from Tools\Options\Media Management, select the option to "use overwriteable media in the target media set before scratch media".