Uf, I'm looking right now and yes, there is _TAG as field.
Problem is, before few days I was making uninstall of old Mp3tagPro app and uninstaller of that app was somehow deleted all Mp3tag settings and all actions with tag sources was deleted. Complete Mp3tag was restored to default values.
And ofc, I was not make new backup and now I need to setup again my M3tag settings.
I can't remember what action I was used in last configuration of Mp3tag but I will look all .mta files and try to find what .mta scrypt use specific tag fields.