The are three ways you can work with studies and references in the outline:
With the right-click menu.
Using drag and drop.
Using the keyboard to cut, copy and paste.
You can select multiple elements of the same general type (study, reference, outcome, figure, etc) and then move, copy or delete all of them at the same time.
To select multiple elements one at a time
Click the first record you want to select.
While holding down the Ctrl key, click each additional
record you want to highlight.
Each record will be highlighted.
Click the first record in the series you want to highlight.
Hold down the Shift key and click the last record
in the series you want to highlight.
All records in between will also be highlighted.