Some tags handled by Mp3tag (according to the Tag field mappings help page) don't exist in the "Edit tag info" dialog:
most of them are tags of the default tag panel, but also :
any reason for these last ones ?
(no change with new install of v2.49 or resetting tags)
when adding many custom tags, the tag panel become a bit uncomfortable : would it possible in a next version to insert separators and user defined labels to obtain a more structured view with tag groups ?
I use the POPULARIMETER tag and it does show in the "Edit tag info" dialog.
Maybe you have to use it (type its name at least once) before it shows in the drop down history.
But beware, and look at this thread. That tag has been compromised (to put it mildly).
Using the latest Mp3tag version 2.49 I was able to reproduce the absence of the tag-field POPULARIMETER in the dialog "Mp3tag Options/Tag Panel" in the sub dialog for adding a tag-field to the Panel dialog.
But there was no problem to add the tag-field name POPULARIMETER manually to the selection list and give this new Tag Panel field a user defined field name.
I've seen this discussion before, maybe in the Winamp forums. Someone says it's there, and someone says it's not. Back and forth. Attached is my screenshot (comparable to yours) which shows that it can be there. I suspect it is not there by default and I suspect the other tags you mentioned are also not shown by default. So that means there is something I did and you could do as well, that causes it to appear in the drop-down menus. Sorry, but I can't be specific about what is that "something". Maybe this thread will help. And DetlevD really knows his way around Mp3Tag, so follow his examples here and anywhere else.
I don't have the same result when adding manually the POPULARIMETER tag: you see it in the LOV after that, but i still don't and the tag seems to be considered as a user-defined one, not as a predefined one, even with a restart. Same strange behavior with the 2 other tags i mentioned.