User defined tag input fields

If I add a User defined tag input field that corresponds to a defined frame in id3 v2.3.0 does the data I put in the window go into that frame?

Suppose I define a user field "Conductor". Put in some data e.g. 'Zubin Mehta'. Write the whole tag to an .mp3 file. Does MP3TAG put that data in the "TPE3" frame (TPE3 is the frame designated for conductor according to the id3 v2.3.0 standard) when it writes to the file or does it put it somewhere else?

You cannot add a user-defined tag in MP3Tag if there is already a defined tag.
User-defined tags are tags that are not defined in MP3Tag.

What other programs show depends of there ability.

Read there ...
http://help.mp3tag.de/main_tags.html

Use the dialog "Extended Tags..." to see and verify which tag-fields are stored into the current selected tag-type to be read.

DD.20140729.0940.CEST

Thanks! That's probably my problem, when I go to 'VIEW" I find that the 'Extended Tags' line is
"greyed out" it won't respond. I need to figure out how to activate this feature.

You have to select at least one track in the files list