A class is a coded software construct, a set of instructions that provides an implementation of a specific behaviour. The language used in Service Definer for a class (as well as for a service) is Java. You can use classes in Service Definer typically for general code re-use, if you want to use an implementation of some specific behaviour in multiple services or multiple service methods.
Examples of this include the re-use, in the same way across multiple REST methods, of specific aspects of error handling, custom logging, or the handling of custom HTTP headers.
Classes in Service Definer are a feature for custom programming only. USoft Service Definer does not offer any automated features to support working with Classes, other than standard query, grouping and listing options. If you write Classes in the Service Definer, these can only be used insofar as you manually code that calls them.
Classes may be accessed in Service Definer from the catalog or by choosing Define, Classes from the main menu.