what does DC mean on my sharp cash register?

You didn't bother to tell us the model number, so I just downloaded the first Sharp cash register manual I could find (which is something YOU could have done for yourself), had a search through for references to "DC" (which is something YOU could have done for yourself) and found that the key can enable double width characters (which is something YOU could have done for yourself).

Here's a link to the manual I found (which is something YOU could have done for yourself).

The reference is on page 39

http://www.rekavo.nl/handleidingen/sharp/XE-A202.pdf

Just do a Google search for Sharp MODEL (replace MODEL with your own model number, of course).

XXX
February 2016