What is the clipboard?
While working on your project, if the page that you're currently working on contains caption boxes with text in them, and you change the Layout of the page to one that contains fewer caption boxes than the current page, the text within the caption boxes that are lost will be copied to your computer's "clipboard".
Whenever you copy text, whether it be by highlighting the text, right clicking on it, and selecting copy, or by using a keyboard shortcut (such as ctrl+c), the text is copied to your computer's virtual clipboard. When Pasting the text, it copies the information that is on that same clipboard, then pastes it to the desired location.
When the text from your caption boxes have been copied to your clipboard, you can simply add a new caption box to your page, and then Paste that text in to the new caption box. This can be done by either right clicking in the box and selecting Paste from the menu there, or by clicking on the More button, selecting Edit, and then Paste.
Note: When you paste the text into the new caption box, it will revert to the default text format. Any changes to the font/size/color will be lost. If the change was accidentally, you can always use the Undo button to change your Layout and captions back to the way they were formatted previously.