Encase 7 decrypt safeboot

Hi All

I'n a newer of Encase software, I saw there are two partitions (C &D) within one drive when I was using Encase 7 enterprise to get a SAFE network preview, however, when I decrypt the drive with a SBD file and open it, there are three partitions show up in the tree pane, I can know the D mapping to the real C partition, and E mapping to real D. The C partition show up in the tree pane looks like created by Encase, and only have "$Extend", "$RECYCLE.BIN", "Boot" and "system volume information" four folders.

I checked Encase paper and internet, not much detail information about it. I just want to know what's the purpose of creating this C partition? is it only happened when decrypt a drive? thanks.