What's the meaning/use of ?mt=8 and ?mt=12 in iTunes Preview and Mac App Store Preview URLs?
MT stands for media type.  You can find known values in the answer to similar question.

Because there’s usually an ID in URL, it may seem odd, but according to another answer to that same question, the mt query parameter helps the operating system to launch the right store in the right app.  For example on iOS, you may see sometimes that links open the iTunes app first and then the iBookstore in the iBooks app.  Other times, the iBookstore is opened right away.  That is why mt should be part of iTunes and why you should probably use iTunes Link Maker.