What is dl_no?

I am wading into the TMS/Wordfly world and working on customizing actions based on email response codes.  In a few tables (most notably t_promotion) I see a column called dl_no, but I can't really find a description of what it is for anywhere. Anyone know what "dl" stands for and what "dl_no" indicates?

Probably a n00b kind of question, but I know you all won't make too much fun of me :)