An addenda is written to explain. If there is an aspect of your application that you feel requires background information or context, an addenda is the appropriate place to write about it. Is there a story behind information on your application that you'd like admission representatives to know about? Topics could include:
Poor academic performance. An example might be if there were health or personal concerns that contributed to poor grades in a particular semester.
Low LSAT score. Were there extenuating circumstances that played a part in a low LSAT score?
Additionally, many law school applications will have questions concerning academic challenges or college conduct/criminal records (which are called "Character and Fitness statements"). Character and Fitness statements address topics ranging from being cited for underage drinking on campus, cheating on an exam, or criminal records.
Addenda formats are different from personal statements or supplemental essays. An addenda is short, factual and to the point. Questions answered include:
What happened?
What caused it?
How did you change or how did you handle the situation?
End with a positive note (lesson learned, positive outcome).
If you believe you may have the opportunity - or may be required -to write an addenda, talk to the prelaw advisor. In some cases, the addenda may not be warranted. If it is, the prelaw advisor can talk through how to write it.