The file operations save and open handle one project at a time. One project corresponds to a model plus diagram information, i.e. everything you can edit within the ArgoUML window.

The model may contain many objects (ModelElements) which form the complete UML description of the system you are describing. All ModelElements might be present on a diagram, but this is not required. Hence, the model that is stored in ArgoUML is independent of the contents of the diagrams. This may be explained by the possibility to generate programming code from the model - you do not need any diagrams for this.

An ArgoUML project also contains all diagram information, i.e. the shapes (presentation) used to represent the various UML ModelElements, their location, color, etc. Some ModelElements appear on multiple diagrams, some on one or none.

Hence, saving and opening projects retains all this information. There is a way to only save the model information though, which is by the menu "Tools" -> "Export as XMI...". This may be usefull e.g. when generating programming code with an external tool that understands XMI.