What does "mixed" file name in my Create tab mean?

In the Create tab, there is a column for the original file name to help you identify the files you want to work on. Sometimes you might see the file name "mixed" instead of a file name.

This indicates that multiple files that were sent in to the system have been merged into one invoice/row and so there is no original file name as you have essentially created a new file which we call "mixed".