_TAG tag function?

Hello there, I am looking through some code and don't know where mp3tag is pulling the data from for _TAG?

For example:

T=4
! Title Case Conversion [3] (Articles, Conjunctions & Short Prepositions to Lower-Case, Except Before or After Punctuation)=
F=_TAG
1=(?<!&|:|;|-|/|!|(?<![A-Z]\\.[A-Z])\\.|\\?|\\+)(\\s+\\b(A|An|And|As|At|But|By|De|Et|For|From|In|Into|Le|Nor|Of|Off|On|Onto|Or|So|Than|The|To|Upon|Von|With)(?\u003d\\s)(?!\\s[\\-\\(\\)\\[\\]{}]))
2=$lower($1)
3=1