How to transfer tag information from one tag format to the other?
Options > Tags > Mpeg defines how Mp3tag deals with tags from MP3 files and Options > Tags > Ape, Mpc, ... is responsible for all other supported formats (Monkey's Audio, FLAC...).
If you want to transfer APEv2 tags to ID3v2 and ID3v1 tags for example, you select the reading of APEv2 tags and the writing of ID3v1 and ID3v2 tags. After applying the settings by pressing the OK button, select the files which should be re-tagged, press Ctrl+X and then Ctrl+V. If you don't want to remove the APEv2 tag format after the transfer, press Ctrl+C instead of Ctrl+X or simply select the tag formats to write from the Mp3tag options, select the the files and click on 
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