Yes, everlasting big problem.
If you want to speak about the tag-field, then you take the tag-field by its name, e. g. ARTIST.
If you want to use the content of the tag-field, then you take the tag-field by its content , e. g. %ARTIST%.
The percent signs around the tag-field name work as a content operator.
But there is also another meaning of the percent signs when it comes to split strings by guessing expressions.
Then the percent signs around the tag-field name is only a visual indicator to the user, that the symbol %ARTIST% is a placeholder for the tag-field ARTIST, which content %ARTIST% will be changed by the following operation.
DD.20110510.0831.CEST