Some examples of this from my Schedules Direct XML file:
<episode-num system="onscreen">8422</episode-num>
<episode-num system="dd_progid">EP00726133.0134</episode-num>
<episode-num system="onscreen">8423</episode-num>
<episode-num system="dd_progid">EP00726133.0135</episode-num>
<episode-num system="onscreen">13947</episode-num>
<episode-num system="dd_progid">EP00001189.5318</episode-num>
<episode-num system="onscreen">4001</episode-num>
<episode-num system="dd_progid">EP03239991.0226</episode-num>
I can't make any sense of those, but maybe Yaobing can...
EDIT: I did find this "Unique description identifier necessary to reference movies, shows, episodes, sports from the programs data. First two digits are alphanumeric and correspond to movies (MV), shows (SH), episodes (EP) and sports (SP).
For shows beginning with EP, the next 8 digits represent the series ID, with the last 4 digits representing the episode id. If episode information is not available, the program will appear as type SH, the next 8 digits as the series id and the last 4 digits as zeros."