DC stands for Day Certain. It's a new product, which you should all have been briefed about, several weeks ago. It's a higher priority version of Deferred Mailings. And it MUST be delivered on the correct day. If it's either early or late, RM get a hefty fine. And could potentially loose the contract.
As it's basically 2nd Class Deferred, it costs more than the old 3rd Class Deferred Product. So there's a lot more money in it for RM. Particularly as it seems to be very popular at the moment.
For some reason, I can't find any details on the RM Web Site. So unfortunately, I can't give you a Link to it. But your DOMS should have all the details about it.