The Apps button as shown in the figure below is used to group custom actions in one place. It is located at the bottom of the Favorites toolbar of Studio.

Figure 7.3. Apps Button

You can add your custom button to the Apps Menu by using the AddItemsPlugin as shown below:

<editor:ExtensionsMenuToolbar>
  <editor:plugins>
    <ui:AddItemsPlugin>
      <ui:items>
        ...
      </ui:items>
    </ui:AddItemsPlugin>
  </editor:plugins>
</editor:ExtensionsMenuToolbar>