When you view or print a song, the program first examines each line of the song. If all of the "words" on a given line are recognizable as chords, that line is determined to be a line of chords.
If a line is not fully transposable, but it recognizes MOST of the character groups as chords, the program presumes that there is a chording error, and will show the suspected error when you try to transpose the song. The line will be treated as lyrics if the song is transposed.
If the program recognizes as chords less than half of the words on a line, the line is marked as lyrics and will not be transposed.