Isn't that the place where some basic settings like serial bit rate, video inversion etc. can be made?
Don't have my DEC 100B right next to me to check, i only remember that mainly periperals were involved.
EDIT: the procedure is described on pages 37 - 52
of the Technical Manual mentioned earlier in this threadftp://ftp.update.uu.se/pub/rainbow/doc/rainbow-docs/pc100tm1.pdf
Try pressing F3 at that stage ... that key is equivalent to "SetUp" on a real Rainbow and enters a special mode during normal operation.
Here it shows