Clicking Join | Merge Documents will combine two or more selected documents into one document. Pages will be added to the topmost selected document in the Separation Workspace in the order that the documents appear in the Separation Workspace.
For example, if a Merge Documents operation was performed to the selected documents in the below image, pages from Check 1301 would be added after the page in Check 1101. The pages in Check 1411 would be added after the pages for Check 1101 and Check 1301:
