The track number is a composite of [discnumber] and [track] ie. [discnumber][track]
[discnumber] is 1, or 2 or etc
[track] is 01, 02, 03 etc which is fine for MP3 files but not for M4A files which don't allow the "leading zero".
Source TAGs: [Discnumber], [Track]
Target TAG???? Please specify. If target is [Track] then you risk to lose original value. You might want to "backup" [track] first to another tag and then proceed to merge [Discnumber][Track] into [Track] action.
action 1. Backup track (optional)
Use action: Format value
Format string: %track%
I think that an action of the type "Format Value" for TRACK would be fine:
Format String: %discnumber%$num(%track%,2)
This should take care of all track numbers, regardless whether they have leading zeros or not.
To split it again, use 2 actions of the type "Format value"
first for DISCNUMBER:
Format string: $div(%track%,100)
this takes only the number that is above 100
second for TRACK:
Format string: $mod(%track%,100)
this takes the remainder of the division by 100
MP4 files never have leading zeros in the track numbers. So I think that actually the intention was to get leading zeros for MP4 files at least in the expression which combined discnumber and tracknumber. But as MP4 would not store leading zeros in the tag, a little trick was necessary.
After establishing what I could do with the Action proposed by your good self, I started messing about a little.
I found that after creating the discnumber/tracknumber format I am able to delete the Discnumber Tag but the combined discnumber/tracknumber is retained.
Is this an accident?
I hope not!
In fact, what has happened is that you have created tracknumbers that are above the value 99. To any player the contents is a 3-digit number. Only you know that the first digit is the discnumber. That is why you cannot simply split the field again with e.g. an action of the type "Guess value" but you have to do it with the more complicated mathematics.