Off-case arguments, sometimes called On-Plan arguments are policy debate arguments presented by the negative in the 1NC. They are generally flowed on a separate sheet of paper each and read before case arguments.
They are so named because they are not directly responsive to the arguments made by the 1AC.
Topicality, although a stock issue, is universally considered an off-case argument, because it deals directly with the plain text rather than the evidence behind it.
References
Off-case arguments Wikipedia(Text) CC BY-SA