We have for example REM DISCID D00DA810 from wikipedia entry. How is it calculated/where does it come from?
As mentioned in the DiscID howto that you can find here : http://ftp.freedb.org/pub/freedb/misc/freedb_howto1.07.zip,
The disc ID is an 8-digit hexadecimal (base-16) number, computed using data from a CD's Table-of-Contents (TOC) in MSF (Minute Second Frame) form.
This document includes a description of the algorithm used to compute the DiscID of a given audio CD.