The SDI outputs are uncompressed "real time" outputs. This enables you to hook up a HDCAM, DVCPROHD, DVCPRO50, DigiBeta etc deck to your XLH1 and capture directly to that deck, OR to a computer equipped with an HD-SDI or SDI capture card (Blackmagic, Kona etc).
The output is 4:2:2 uncompressed, which translates to approx 1.4 gbps for HD and 270 mbps for SD. Now remember that DVCPROHD uses 100 mbps and DV 25 mbps and you see the difference in bitrates!
If you shoot to tape that will be either DV or HDV. That stream you can transfer over FireWire.