A category is a independent “tag” for an object. Categories make it possible to organize or group parameters, expressions and other blocks.
To assign a category to a block, either
Click the Category column of the blocks window. Either enter the name of a new category or choose one from the drop down list.
Edit the block, and assign the category in the properties page.
In the blocks window, there is an option to search for blocks of a given category. You can also sort the table by category by clicking the Category column header.