it would be included as Sega of America or Sega of Japan because SEGA is the holding company. There not companies on there own. DigitalRex may be one of those companies within the 'SEGA' listing.
here have a look SEGA has reserved a massive space (in comparison to last year) for E3 2004. This picture is from the E3 site. I just put the sega logo and red line around the location. Hopefully with this big space they should have some elaborate Shenmue 3 debut.
Yeah that seems like a good spot and you say they have reserved all that? Inside the red line you made it says 'Concourse Hall'. They've reserved the hall! Maybe this is for a big announcement ;) wink wink shenmue 3.