Smalltalk systems have traditionally provided class categories to help organize classes into related clusters. They had no semantic value, and were not represented by objects. In VisualWorks, categories have been supplanted by packages.
Unlike traditional categories, VisualWorks packages are real objects (instances, surprisingly enough, of the class PackageModel). They provide the organization feature of categories, but they are also the basic archival unit for the Store repository.
When developing application code, you should create your own new package to contain this work, rather than use an existing package. To create a new package, make sure no package is selected (Ctrl-click to deselect, or just select Local Image) in the System Browser, then pick Package | New... and enter a name in the dialog.