The Multiple Selection button is available only when it is allowed to select more than one element.
To select more than one element
Click the Multiple Selection button. The Selected elements area appears on the right side of the element selection dialog.
In the model tree or list, select one or more elements at a time by holding the Ctrl or Shift keys.
Move selected elements to the Selected elements area.
To return to the single selection
Click the Single Selection button.
When more than one element is selected in the tree or in the list in the multiple selection mode, the selection will be reduced to one element (the first one based on alphabetical order), when switching to the single selection mode.