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DMT is just from the company name Diamond Machining Technology.

325 is for 325 grit, the size of diamonds on that hone, so you will find many different products if you search for "DMT 325".

(C) Diamond - Coarse
Quickly sharpen a neglected edge (325 mesh, 45 micron)

It's more or less the standard for lapping by people here, but Atoma is a good alternative or any cheap Chinese coarse...

When lapping stones you do not want to use it dry, the build up of loose grit rubbed around on the DMT will eat the substrate that holds the diamonds.
All diamond hones and some ceramic too is more...