Enterprise Architect has many default Toolbox Profiles, one for each of its inbuilt diagram types. These define the Toolbox that is displayed, by default, every time a diagram of a specific type is opened or brought into view.
If you want to replace one of Enterprise Architect's default toolboxes with one of your own (for example, if you have your own version of the UML::Class toolbox that you want to be opened every time a class diagram is opened - as long as your technology is active) then include a RedefinedToolbox clause in the Notes field for the diagram properties of your Toolbox Profile diagram. For example, the profile diagram's Notes field could resemble the following:
RedefinedToolbox=UML::Class;Alias=Class;Notes=Structural elements for class diagrams;
This states that the profile is the new Enterprise Architect default for all UML class diagrams. For a list of inbuilt diagram types, see the names of the toolboxes.