Diagram (element type)
The diagram element represents a graph. It consists of nodes, edges and compartments (all of them are called as diagram elements). A node is usually represented as some geometrical shape like a box or an ellipse. An edge is a line which connects two nodes. A compartment is a special type of node which can contain other nodes within.
When you double-click on the diagram in the repository tree, a diagram document is opened allowing you to view or edit the diagram. The only exception is the workflow diagrams: in BioUML web edition by default they are opened as analysis documents, where you can set the workflow parameters and launch it. Nevertheless you can edit a workflow if you use the "Open diagram" context menu action.