The DISCNUMBER field is
mapped to the TPOS frame in ID3v2.4 tags which is
specified as follows:
QUOTE(ID3 tag version 2.4.0 - Native Frames)
TPOS
The 'Part of a set' frame is a numeric string that describes which
part of a set the audio came from. This frame is used if the source
described in the "TALB" frame is divided into several mediums, e.g. a
double CD. The value MAY be extended with a "/" character and a
numeric string containing the total number of parts in the set. E.g.
"1/2".
Basically, there is no guarantee that foobar2000 will let you store non-numeric data in the DISCNUMBER field when using ID2v2.4 tags.