Prior to Registry software release 1.0 around May 2006, each part had a single "Type" such as "terminator", or "RBS" selected from a small list of part types. The letter in the part number was supposed to match the part type. This had two problems: First, many parts seemed to belong to more than one type and, second, other parts seemed to belong to no existing type. When the user clicked on a part icon in the icon bar, she was taken to a table that held all the parts of that type. These tables became too long and undifferentiated.
The strongest solution, not taken, would be to eliminate part types and categories all together and use a text search instead. This would leave the program without a format for the resulting table. It would leave authors with no way to author information about a category of parts. Look at cell-cell signalling parts for an example.
This has been resolved by creating an explicit tree-structure of categories that represents a larger variety of part types in a heirarchical form and allows specification of table structure at each point in the tree. Furthermore, a part may belong to many categories at the same time. This table will be a dynamic user interface supported by the Registry.