From some quick searches, I see the difference is the size of the granule.
I put together a batch of mustard this morning - the double batch called for 2T. kosher salt. I only had Trader Joe's sea salt on hand and used about 1 1/2 T. of the salt (thinking it was SO much salt!).
I plan to make another double batch in a couple days and want to adjust the salt. Ideally, I'd like to use 1 T. sea salt.. instead of 2T kosher, but that may not be enough??