These two options in the Review tab are often unnoticed. Here is what they are for.
If you have an original document and another revised (changed) version – but you did not enable Track Changes before changing it – you need the COMPARE option. It will take both the documents, compare them and create a third document showing Tracked Changes.
If you have sent a document with Track Changes ON to many people and all of them send their revised version to you. Now you have multiple documents with each containing tracking by a DIFFERENT person. In this case you use the COMBINE option – two documents at a time to MERGE the track changes. Of course, changes are not accepted – just accumulated. Try it!
This requires Word 2013. Just choose Simple Markup from the dropdown for track Changes. The tracking is hidden. The document is shown as though all changes have been accepted. Wherever there are changes, those are shown with a red line in the margin. Click on the red line to view all tracked changes. Simple and Effective.