You specify categories to make your effects shaders accessible from the preset manager and Nodes menu in the render tree. You can create your own categories, or you can add your shaders to the existing Softimage categories, but you can only specify one category for each shader.
For the curious minded, the realtime shaders category and subcategories are defined in the RTShaderCategories.js file located in the \Addons\RTUserTools\Application\Plugins folder. You can create a similar user plug-in to add any other shaders you want, and to anywhere in the preset manager and Node menu that you'd like. You can even start from a clean slate by setting the environment variable XSI_NO_DEFAULT_SHADERS.
If your custom effects file does not include any categorization, you can edit your effects file and enter the category string directly. To do this you can use the Softimage shader code editor (choose View Rendering/Texturing Shader Code Editor from the main menu), or you can use any text editor for that matter.
If you want to place your custom effects shader in a different category, you can edit the shader and enter the category string from within the render tree. To do this, drag your preset from the preset manager into the render tree workspace. Then, right-click the node and select Edit Shader to open the shader code editor.
Once you have finished editing the effects file in the shader code editor, click Validate to check your code and then Apply your changes. To update the preset manager you may have to restart Softimage.
string Category = "MyCategory/MySubcategory/MyOtherSubcategory";
You specify a subcategory using a forward slash (/) preceding each nested level (of subcategory). You can nest an unlimited number of levels deep, although it is recommended for clarity and usability to only use up to two levels of depth.
To group items, you indicate group membership by the @ symbol followed by a number that identifies the group. It is best to use multiples of 100, to give maximum flexibility for interceding groups. For example, suffixing @100 to the Category name for three out of five shaders means that those three will appear after the other two, and each of the two groups will be automatically sorted in alphabetical order. If you then added another set of shaders with the group indicator of @200, those will appear at the end. Each grouping is indicated in the preset manager by a standard menu separator.