RTL means a retail-packaged drive. It comes in a colorful box, safely packaged, with some software on a CD, and four mounting screws.
A bulk drive does not come in a box, unless it's packaged in a plain white/brown box by the distributor or the retailer selling it. Bulk drives have different plastic bezels without the BenQ logo.
The suffixes have the following meaning:
-0C0 beige, retail, with an additional black (bulk) bezel
-0C1 beige, bulk
-0C2 beige, retail
-0C3 black, bulk
-0C4 black, retail
You can see the difference between retail and bulk drive bezels here:
The hardware is identical for all DW1620 and DW1620 Pro drives.
Retail drives come with B firmware, bulk drives - with G firmware. You can crossflash bulk drives with the B firmware using WinDWFlash.
Again, the hardware is identical, so besides the black vs. beige bezel color the only important distinction is in the retail vs. bulk bezel appearance, if you care about such things.