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Wayne hit on it,
it is a plate to maintain your stone. Quite a few will use a flat surface around 400 grit wet dry sandpaper.
This can be adequate if you are not doing a lot of honing.
The DMT8C...

Loaded question.
Are you doing restores/eBay specials?
Are you just maintaining an edge.

Here is a great read to help narrow down many of the questions you will have to start.
Give it a look...