There are various places were we have the favorite button. Please let us know at what page you are talking about. It is a bit hard to know which particular button you are talking about. Thanks! :)
I found in between, it is the "Mark" button, to mark the song currently playing. It gets active when the radio station transmits song names.
Yes, exactly the TAG button you showed below in the image gets enabled when there is actual song information available.
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