I have recently joined the RPGA and when trying to set up my password I found it wanted a DCI number. I have a 10 digit RPGA number so I used that. I wanted to know what the difference is.
In terms of the number you have in front of you, there's now no difference. RPGA and DCI numbers are now interchangeable. In terms of programs, the RPGA is WotC's organized play arm in charge of all things D&D. The DCI is WotC's organized play arm in charge of all other games that WotC has organized play for (which these days is mostly Magic). Up until a few months ago, the two were separate and distinct, and if you wanted to participate in both organizations you had to have two different numbers. However, the numbers have now been merged and you can participate in any sanctioned activity from either the RPGA or the DCI off of a single number.