Overlay mode is best for comparing visual layouts, such as two drawings with markups. The selected files must be different versions of the same document. The two versions are superimposed on top of each other with additions and deletions colored to highlight differences between them.
View two documents next to each other, for comparing or referencing.
Note: If the two files are not different versions of the same document, then they must be in the same Collection together and the most recent version of each document.