No, MP3tag does not calculate. It simply detects that the directory has changed.
So if you do not sort the files by folder but, let's say, alphabetically by title, the folder of a collection of more than one folder will get several 1s in one folder - everytime a new folder is detected, the counter is reset.
MP3tag does not know anything about discs or whether files belong together and MP3tag does not remember anything from the previous file.
The automatic only has to be used, if ...
the files of a multi-disc-set have to be stored in several folders
the discnumber should be indicated in the field DISCNUMBER (which hardly any player shows)
the discnumber has to be set after the files have been created initially.
It is also possible to store the files in just one folder but to indicate the disc number as part of the track number.
So track 1 on disc 1 would get as track number 101, track 2 would be 102, 4th track on disc 2 would be 204.
This numbering scheme works with up to 99 discs and 99 tracks per disc.
The track number is shown by more or less any player, so you can even sort the files on players with rather basic functions.
The offset can be set during extracting files from the CDs without the need of filling the field DISCNUMBER.
If you still want to have data in the field DISCNUMBER, you can extract it easily with an action of the type "format value" for DISCNUMBER and the
format string: $div(%track%,100).
No auto-numbering needed, these functions are already there, so you can bridge the time until the function gets implemented.
One never knows what the future brings ...