Type of project data
Different templates are available for different project data. The drop-down menu there divide project data into four main types:
Project - The entire project. You will see element templates mainly for querying details of diagrams in project.
Diagram Type - The available types of diagram. (e.g. Use Case Diagram, Business Process Diagram)
Model Type - The available types of model elements. (e.g Use Case, Class)
General - Mainly for listing legacy or obsolete element templates. Normally you don't need to deal with templates under General section.